I don't think there's anything wrong with looking and appreciating someone attractive, but I think it's indecent if you think about having sex with that other someone while being in a relationship.
Fantasizing is also harmless. Big difference between thinking about something and doing it.
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If young Brad Pitt or Hugh Jackman walk by, you're supposed to think at least briefly about sex with him. I'm not jealous. That's what you're supposed to do.
Well, I don't. I do appreciate handsome men, but I don't think about having sex with them. If I do think about sex, it's either with my boyfriend (if I am in a relationship) or with someone I'm interested in dating. Therefore if I'm in a relationship, I'm not thinking about sex with others just because I'm not interested in dating them, since I'm already dating someone else. But that's just me.
@Starcluster I still get the primal urge for it even after marriage. Girl twerks in front of me and caveman kicks in. But I can resist it you know, just like I might smell pizza and I can resist it.