I think your gut is right. The question is, is the duty sex worth it to you? Does it satisfy you at all? If yes, then don't kiss her. Skip the foreplay unless you really like going down on her. If you like that, and it appears that she does since she lets it continue longer than other forms of foreplay, then do it. If not, just use some lube, play with her in ways that please you. Do the deed, talk dirty if you like it, and generally whatever else pleases you. Perhaps she will join in one day. If not, you are still getting some sex.
If the duty sex is not satisfying, then don't do it. Your self respect is important. Of course, I'm a big fan of The Talk™ and I'd force her to listen while you explain exactly how this makes you feel like shit and wonder why you two are married since you can be friends and respectful people without being married, but marriage is the culmination of both friendship AND intimacy, you both deserve both.