I just don't belive that would be their future. He was ready for marriage.. urm, marriage is only something formal, a piece of paper, love showed by law. He loved Anya and she loved him.. I don't think marriage would change that, because they kinda were in marriage already.. loving each other and living together.. marriage is just a thing to show the others their love.
Only kind of. It's a lifelong committment. Xander saw how well that worked out for his parents and how much that simple piece of paper and mere law damaged their relationship and his childhood. Marriage is a heck of a lot more than loving each other and living together and that is what Xander realised. He could do the loving and living together bit but marriage, while being a continuation of the same, is also altogether different. And it doesn't matter if you don't believe that would be their future; I don't either.(Well actually I think it could have been a possibility had Xander gone through with it when he felt the way he did.) What matters is that Xander did and that is why he was entitled to do something for himself, and ultimately for Anya, and not going through with it.