My wife won't swallow, even though she says she likes the taste of my cum. It's not a deal-breaker, but I had a girlfriend who did swallow and that was AWESOME. I still fondly remember and fantasize frequently about those blowjobs, even though my wife gives better head overall than my previous girlfriend did.
I do agree that it's mostly mental. My wife will let me cum in her mouth and lets me stay there until I finish cumming. Physically, that's the really important part. But it IS a bit of a letdown when she then gets up and spits it out. Sort of like a rejection, if that makes any sense? When my previous girlfriend swallowed, I felt a deep satisfaction and sense of acceptance that I only experienced with her.
I used to give blowjobs, and I've always liked the taste of cum and the experience of swallowing. I don't really understand the repulsion some feel — but I also know that I don't HAVE to understand it. I just have to accept that it's a hard limit for my wife and move on.