Sera is... complicated. What sex is a biblically accurate angel? The artist insists in calling Sera a "her", but has always been drawn (and acted as) a very muscular male. Whatever. I read it because the whole series is hilarious, and the bit about "the sex of angels" just adds to the hilarity.
This is actually the first ever time I've seen these two having honest to god sex. The entire time I though they had a sexless, but very happy, marriage, on account of thinking that Sera was functionally asexual and never going beyond hugs and handholding. But nope, not only Sera is perfectly capable of having sex, turns out she's also pretty good at it too.