I've been having a similar issue as well. I think in part mine has to do with the birth control pill that I'm on. Its a pretty common side effect. For the first part of our relationship I wasn't on it but after I started my drive has gone out the window. But I don't think its only caused by that so I don't want to blame all of it on the pill. Also, before I met my FI I was in really good shape. Running and working out with a trainer etc. I had a good eating regiment and I was really proud of the way I looked. Once I met Michael and I moved to Vancouver my workout/eating routine went out the window and I got a bit soft again. Since I feel gross about myself I find it hard to feel sexy and want to be intimate. I can't remember whether I read this somewhere or if I saw it on tv...but I remember a counsellor saying that for women their sex drive is influenced by a million little things, diet, body perception, emotional intimacy, stress etc... anything can sort of throw it off. Usually though its issues within ourselves that we need to workout.
For me too, because we live together I find its easy to be lazy, and just put it off. Its hard with work, studying, workingout, keeping the house from falling apart etc.. to have alot of extra time, so its easy to say I'm tired and I don't feel like it...lol. Actually my friend was saying she never feels like it anymore but just does it anyways, because even if you don't really want to at that moment, you never say to yourself "oh boy I wish I hadn't done that" once its over. Chances are you are happy...so maybe the trick for me is to just suck it up and do it. As well as continuing to try and workout and get back on track with my body, because if I feel good it will show I think.