i never saw his leaving anya and wanting to stay with her as a sexual thing. he loves her dearly, and wants to be with her, but think about it. not only was he shown his literal worst nightmare the day of the wedding, but the time leading up to it was pretty shitty, too.

let's focus on what xander actually is afraid of. is it anya's family? shit no. he clearly gets along better with demons and hellspawn way better than he does anyone else. is it anya herself? obviously not, he adores her, even throughout the rest of the series.

it's his family. his parent's abusive relationship is a gag throughout the first four seasons of the show, but in the wedding episode we see his family without he easy gags, without the awkward hilarity of being the friend hanging out in the basement while listening to the argument a floor up. much of xander's development had to do with overcoming his family... but at the wedding, there's alcoholic uncle rory hitting on his underage substitute little sister, and anyone else with two legs and tits. there's his parents sniping at each other every chance they get. there's his dad retreating behind booze and cheap shots at his mom.

what's the nightmare? it's not that he and anya grow old and have shitty kids. it's that xander ruins anya's life. nothing could devastate him more than knowing he took someone who genuinely made his life a better place to inhabit, and dragged her down to his level, the level that seemed inevitable based on his upbringing.

i get not necessarily loving xander, but as a person with low self-esteem who married a wonderful lady, trust me, i sympathize with his position. it's extremely human, and not all that uncommon. xander's failing was one of the things that spurred me into realizing what a wonderful life i could have, and that's kind of the point of these characters. they make the mistakes that we don't have to. i'll always love xander dearly.