Is it awkward for a guy to think his wife/girlfriend's sister is prettier than his wife/girlfriend?

He can think whatever he wants. It's how he acts that matters.

I don't think it really matters. Looks don't win it over for me, and I certainly wouldn't start dating someone's sister if I was initially interested in her at first. Too incestuous for my taste.
Unless she cheats on him, he will stay with the girlfriend.

This goes way beyond awkward. To admit any of that, you're signing your death wish

I’ve experienced that before. It made me feel guilty but I never acted on it.

No. If she's prettier she is. We just don't necessarily want to "feel her up"

Probably, but there are other, better, criteria, that make for a good partner in life!

I was seeing a girl at uni for about two months when she decided to invite to meet her family . When we got there the last one I get to meet is her younger sister who it turned out I'd had a one night stand with some months earlier . That's what you call awkward.

This shouldn’t be the case so I’d say yes

Only if either of them EVER find out.,.,., Carry that to your grave....😷

If prettiness matters that much why did he marry the 'ugly' one?

It's only awkward if he says it out loud.

[media=https://youtu.be/txTEhgReZUA]

If he admits it to you. It's kind of weird.