In my opinion, there isn't just one thing happening here, but a multitude of issues playing out.

1. It sounds like you've both moved from having sex for fun and bonding without any real expectations to feeling anxiety, tension and rejection. These mental hurdles can make enjoying sex difficult and awkward and often are indicative of similar issues in your relationship overall. Personally, I think you guys need to reconnect first. Go on dates to the movies or start a pillow fight. Skip out on housework and go get an ice cream. Play together. Rebuild those bonds.

2. There is also a physical component. You might try things like Yours and Mine to not only alleviate the pain, but maybe even heighten enjoyment.

3. Communicate what your needs and wants are outside the bedroom. Don't try to wing it when you're already in the middle of things. You've said you don't really know so talking through things might help. Start with exercises like "I love it when you ..." and "I wish you would do ... more". And instead of just deciding to have sex, say, "Tonight we should try ..." (Not to mention there is a verbal component to sexuality ... ahem ... why he wants to hear you ... and that can also assist in getting things going). Be more exploratory, hubby should get out his goggles and swim fins and not come up for air and maybe you make a trip to Victoria's Secret (or some other exciting places )

So, to sum up, take some time to address those tensions in your relationship that are rearing up strongly when you talk about sex. Fix them outside the bedroom so they aren't in the way when you are in the bedroom. Consider marital aids. And communicate, communicate, communicate. Talk it through and work together without judgement or rejection, but using teamwork, compassion and love.