I've been single since almost 2 years before my AS diagnosis, so my sex life is nonexistant. But - for about the last year we were together, I had a really hard time after/during sex. Either my SI joint area would become really painful, or afterwards I couldn't walk because my hips were so bad, or my muscles would be in spasm and not relax. And we weren't trying any acrobatics either...
I have to say that I'm almost thankful that I don't have any pressure to have sex. It would be so stressful to think about having to because it seems like it would be SO painful. I can completely understand the not wanting to make your partner feel rejected, and I really wish you the best of luck and hope you can find a way to be intimate and comfy.