I think jjarvis has given some very good insight/advice
Basically, it is a problem, because your fiance obviously would like to have sex, an you don't feel like it, hence --> conflict.
I think, whilst it would be pertinent for him to be patient and understanding, the position of "not wanting sex" is not superior to the position of "wanting sex" when in a relationship.
It is your right to refuse sex, but not your right to say that your partner must be happy about it. And he's perfectly justified in feeling unhappy about it, and trying to talk to you about it.
IMO, if you were to completely ignore his needs, that would make you selfish. You can't help it you don't feel like sex, but you can choose whether or not you're gonna do anything about it. IMO, if your bf is unhappy then you should be trying to do something about it.
What I'm saying is, don't just lie down and take it when you don't want to have sex, that would be kind of awful
I'm just saying, try and work out why you don't want to have sex, communicate with your partner, and try to address the problem.
I've had periods where I haven't felt like sex. But I have done something about it. It's really really tempting, to just think "oh I'll wait for this to pass" or think "this is embarassing" and just abstain with no explanation to your bf.
But this results in hurt and confusion for the other person. They feel helpless and unappreciated, unsexy, undesirable etc. All that is besides actually missing having sex with you! I know, because I've also been in the position of being the one who is being denied sex.
You have to talk about it, get to the root of the problem, and address it, or at the very least reassure your partner. When I didn't feel like sex, I found the best thing was jut to build it up. I kinda felt like all we did was have sex, and it was just really routine as well - and it really put me off! So, I said to my boyfriend, can we do things differently? Can we try and just you know, watch a movie, can we do something romantic, can we build up slowly, can we try more foreplay? I feel like all we do is just have sex, and I don't always feel like having sex, I don't feel happy.
All it was is that we had gotten a little bit too used to each other, and we addressed it and moved on. And sex got a lot better, and as soon as I felt it wasn't expected of me, I started to want it again.
Anyway, I don't know what your reasons are, but those are my thoughts on the issue.