"Bonito" vs "Guapo"
So in Spanish, guapo is "handsome" and this has a more masculine connotation than "bonito" does. So, while bonito is not poor grammar, a man might not appreciate being called "bonito". This doesn't seem to be the case in Portuguese, where "bonito" doesn't seem to have the same, more feminine (for lack of a better descriptor) connotation. Is this accurate? Or is there a better word to use?
Bonitão is a commonly used word. I think it shows that bonito is used not only to describe feminine beauty in Portuguese, but also covers the meaning of "handsome".
Standelf is right. Bonito is used to describing men and women. Bonitão is used to describe someone really handsome. There are also lindo and lindão for this purpose.
Semantically: bonito < (bonitão = lindo) < lindão.
My understanding: bonito < (bonitão = lindo = lindão).
Informally bonitão, bonitona, lindão, and lindona are accepted. Actually, it is the way most Brazilians speak. But those forms are not really grammatically correctly. Bonitíssimo, bonitíssima, lindíssimo, lindíssima are the correct augmentative adjectives forms in Portuguese, respectively.
Português vs espanhol. Não é comum no Brasil. Mas está nos dicionários.
If you are really self-conscious about your straight identity, to the point in which you display some fragile masculinity, you will avoid calling other men bonito, lindo or atraente, and instead call them pintoso. But that's on you and not on the receiver.