Well, I'm vegetarian, and I probably wouldn't swallow if my partner was a meat eater just because of the taste - it does affect it! But the actual swallowing wouldn't be a problem. I mean, it's hardly sustenance, is it? And most people's objections to eating meat, at least mine and the majority of vegans I know, centre around animal welfare and health reasons, so I'm not really sure what the problem with swallowing during recreational sex would be.