I'm sure someone can tell you about the Greeks. I'll try to explain the Roman view.

Roman view on homosexuality is quite complicated. First you have to realize that the Romans had a slightly different way to defining the gender of a person. There's the natural element of man and woman but the cultural point of view is very very strong in Roman society. There were certain things expected from a man like being brave, honesty, such things...

If you did not fulfill this as a man you were easiliy stigmatized as a woman. When you look at some of the most famous examples of senatorial propaganda like Tacitus, Suetonius or the Historia Augusta you'll noticed that emperors disliked by the aristocracy are always marked with female attributes: soft, greedy (yes greed was a female thing!), love for arts and many more things.

The same worked the other way around, women active in politics recieve comments like "although she was beautiful and blablabla, she behaved like a man".

Why do I tell you that? Roman view of homosexuality was generally defined like that as well. It was ok as long as you are the "penetrator" (then you are a man and use your male power, are active, dominant) but the one who is disliked is the passive one (who is not manly but female, or slavish).

So part of the propaganda against Caesar was that people said he was Nikomedes' boy when he was young So they said Caesar is not a man but is weak. No one would have taken offence if Caesar would have taken a slaveboy because then he would just satisfy his male needs.

So Romans don't take offense in general it just depends on who is doing what.