You may enjoy the kids when they are older, but only if you're able to create a positive relationship with them now. If you know for a fact that you have negative feelings towards them and aren't good at hiding them, then also know for a fact that those kids feel it. They aren't going to turn 16 and magically like you if they've spent the past 10+ years as your adversary.

From the ages of the older children involved, I get the impression that you are old enough to be established and financially independent. If that is the case, why not leave well enough alone for now. You enjoy him and his time, but not his children. You can maintain your own residence while he maintains his (even if one ends up being the primary residence for both of you), and on weekends when he has his kids, you each reside at your respective homes. That way, you are just "dad's gf" to his kids. Even if, legally, the two of you marry, his kids don't have to know, no one's feelings need be hurt, you have certain weekends to hang w/ your girlfriends while he cares for his kids, and you don't have the stress of "redcindy, I accidentally broke your thousand dollar china playing tea w/ my stuffed animals."

I can really identify with your problem and I would think twice before marriage. I have been married to my husband for two years now and he has 3 kids (8,9,&10). I have one myself (10), and one child is all I ever wanted. I don't love kids like i feel I am supposed to. Bunches of kids together at once make me anxious and uneasy. My son is very well behaved, but my step-kids not so much, especially his daughter. I didn't think twice about marrying him because I truly love him and didn't think anything could get in the way. We have also known each other since we were kids and I felt like after being friends for so many years we would be perfect together. We have one of his kids full-time ( as well as mine), which works out pretty well, but every-other weekend is a nightmare for me. I often have to leave the house when his kids get here (which upsets my husband) and his daughter is a devil to me (which my husband fails to see). All four kids at once is just too much for me and it's driving a huge wedge into our relationship. I just think there's something wrong with me that I don't like kids more!
So, I guess what I would tell you (based on my experience) is that it does not get better, but I am only 4 years into it (married 2). If I could do it over, I would have continued dating my husband until I knew that I could handle it, which is what I think you are considering doing. Go with your instinct and work on the relationship with the children as well until you feel comfortable.
Sometimes I feel like I'm alone in the "not really loving children" thing. I feel better now that I have read your post. Good luck, and if you come up with a way of dealing with your feelings and making your relationship work, please let me know!

I can also identify. If you feel that way that's just how it is. I do a lot of hiding in my room or find other things to do outside of the house going shopping going to the library etc. while they are here I love my dh and I'm not going anywhere but I don't want to parent his kids. I raised mine and it's time for me.