After 12 years of catholic school, I don't recall anything specifically, although I bet some would argue the commandment 'you shall not commit adultery' might cover that.

What I do know is there is a ton of stuff the bible recommends that Christians are happy to ignore. At least anecdotally in the Catholic community I grew up in in NE Philadelphia.

Not an explicit rule, but Jesus is alleged to have said some stuff about how you're committing adultery in you're heart if you so much as look at an attractive person and lust after them.