Hi there,
Congrats. on the pg. best wishes for a healthy one.
I agree with what everyone has said, I don't think either of you are being selfish. I don't know you DF but if she is anything like me then the mere feeling of being pressured to DTD when I'm not up for it turns me off like a tap. I would definitely let her know you are attracted to her and make her feel special, do some extra house work to lighten her load, do these things without any expectations of her. You may feel like it's all a lot of hard work just to get what you want, but it will have far reaching consequences now and in the future.
I used to emotionally get stewed up just worrying about the fact that I wasn't DTD as often as DH would have liked, for me I think it was a combination of a few things like tiredness, hormones, worry, m/s, changing body shape and spotting. I really love my DH so it wasn't that all of a sudden I didn't want him, it's just that DTD was the last thing I wanted for a while.
It has been the same with all three of my PG. and may I add that the frequency dropped off for a few good months after the bubs were born, once again hormones, sore bb's, need for some personal space, tiredness, soreness when DTD all flattened my desires.
Keep the lines of communication open and try not to take it personally.
I hope things work themselves out soon.