You should have stayed home. No matter how well you dressed, your derisive attitude did was not hidden by it. You went into their church to do this. This wasn't on the street or outside your house. You invaded their space. Your description of your families' behavior does not do you credit. Next time do everyone a favor and stay home.

I think it would have been fine for DH to say no, this doesn't sit right with me, and refuse to go entirely. He doesn't have to go along with every cockamamie decision BM makes. 

It sounds like you and DH behaved like children. You went into the situation determined to make your disdain known. The better option would have been not to attend.  I doubt your DH would have been able to stop the baptism, even if he had refused to go along with it.  However, your DH can still help to shape SD's beliefs by offering her alternative beliefs and choices as she gets older. There was no need to go into a religious sanctuary and poke fun at it.

You embarrassed yourself.  You were rude. You should have stayed home or at least controlled yourselves. The people at the church knew nothing of your issues. They didn't deserve your intrusion.

OP, I get your frustration in this situation, and this being your first real dive into BSC religious stuff, why you and DH were on the backfoot and behaved as such.

BM here has the boys in a backwards community church. When I was trying to get along with BM, I went to 1 sermon. It was around the 4th of July, and the sermon topic was "freedom". This is important to note, because somehow, this two-bit snake oil sales, man of the cloth weaved in AIDS is punishment (except, specifically, for those poor raped girls in Africa - he didn't want anyone to think he was coldhearted I guess), the "homosexuals" are going to Hell for their fake marriages, and the HPV vaccine causes premarital sex (that one led DH to ask about the boys' vaccination records). To say steam was streaming out of my ears is an understatement, and I'm sure the look on my face said a whole, whole lot.

DH has made it ABUNDANTLY clear to BM that he won't participate in any baptisms or other formal indoctrination into church. He'll go when the boys perform at Christmas, but he winces during the sermon portions. He'll go to fundraisers for specific trips the boys are going on IF he agrees (there was a mission trip to Haiti that BM wanted OSS to go on, and he said absolutely not since international mission work is BS 95% of the time). Once, BM dropped off baby bottles that you were supposed to collect coins in for the local crisis pregnancy center. I'm very pro-choice, and I'm not about to give a dime to a place that lies to their clients.

Point is, BM KNOWS well enough that DH doesn't agree. She KNOWS how he reacts (usually with as straight a face he can make, but will answer honestly if someone answers him). If she doesn't want DH involved, she doesn't have to invite him (or the boys don't). Your BM knew DH is Jewish. She knew he wasn't Presbyterian. She knew she was doing something that he wouldn't agree with, but she did it anyway and invited DH. 

Should you and DH been respectful of the people there? Sure, but a snigger isn't the end of the world. I'd have not cared about my behavior after the political comment about Isreal. Use the pulpit to preach love or shut up. If you don't use it to preach love, then don't expect a loving reaponse in return. I also question the authenticity of any church who baptises children without talking to BOTH parents first IF both parents are going to be agreeing on the baptism. Especially if the other parent had never been there before and there was no previous conversation with that parent about their want and ability to raise a child in Christ. In my opinion. The religious leader of that church isn't worth the price of the robes they wear as a holy man if he didn't bother to even make sure that all parties were comfortable. He put your DH in a position to lie, and that's unfair to anyone visiting your church. Very poor form.

Overall, nothing about this event was holy and spiritual, and BM put DH in a no-win situation. If he didn't go, he's helping BM alienate the kids. If he does go, he is put in an uncomfortable position of lying. He should have told BM that he would attend in support but wouldn't participate, but in the heat of the moment when you already feel uneasy, it's easy to see why it happens. BM shined a spotlight on you all, knowing full well that this isn't your family's scene, and everyone got to see "the warts". Too bad.

I stopped reading after “DH was whispering alternative lyrics to the song”. And the whole church heard it. And all this giggling. And they didn’t appreciate it. No kidding!  I can’t even... I am Jewish, but I attend all kind of events in churches due to having Christians in my family and Christian friends. I can’t even imagine. Adults behave in this manner, wow. 

BM maybe isn’t wearing a bra but you two lacked class completely. If you aren’t in support of the event or this particular church, there are other classier ways to conduct yourself. It doesn’t need to be “one’s scene” to behave appropriately 

I'm not religious, so I am going to defend you here and say this pearl clutching is a little ridiculous. It's not like you were talking loudly and kicking your muddy boots up on the Pew in front of you. 

The super religious type who condemns someone for giggling in church is probably beating his wife and kids. Sorry, I just find most of them so hypocritical it's hard to find much sympathy for the person wringing their hands over a little giggling.

What happened happened. It's time to think of how to act in the future when blindsided for events. Not just religious events, but all the school and sport events the future will bring.

I wouldn't worry too much about religious events. BM may stay with this denomination or she may change to another, but baptism is baptism and is usually good from one church to another and not redone (yes there are theological exceptions, but they are not common). If BM stays with this church the next significant religious event (confirmation) is years away. You have plenty of time to decide how to deal with it. 

BM is going to take them to church (or not) on her time. DH and you are not going to (or are going to) take them to church on his time. The kids will be fine. They will figure it out. My kids, from an intact marriage, with both parents moderately religious practicing, ended up one agnostic, one fervently religious.

They know, they get non members at there events.  They get people of no or other religions.  This is just part of life.  Remember All those divorce people made the same promise , also all those people having affairs were standing ny you. 

It’s not clergy’s responsibility to make sure dad is ok with religious upbringing of his kids. If dad had concerns, he could call the church and ask ahead of time. He could also tell pastor that he is in support of the ceremony but can’t make personal promises to raise his children in church as he isn’t even Christian.

If he chose to not address anything and just passively follow along then he should have been polite, at the very least, not making mockery of other people’s faith. Personaly I wouldn’t be able to make a promise to raise my child in Christian faith but I also would talk to pastor ahead of time and also would behave appropriately attending church, regardless for what reason. 

I was raised religious, my parents were pastors/missionaries.  I stayed in the faith up until about 5-8 years ago when things drastically changed for me and my views.  I do not agree at all anymore with organized religion.  I find a lot of stuff I found normal then to make me cringe and find it very uncomfortable today. I would honestly classify myself as a Deist today. 

That said, regardless of how much eye rolling would be going on in my head today to sit through a sermon -say I had a baptism of a friend/family (which I did recently)-I will act with the utmost respect as to not insult others.  No reason for it. I agree, you can't claim you stay classy and bm didn't and then both you and him acted this way...also showing the kids disrespect of others and their beliefs.