Rule #1: Don't tell him that he's too feminine.
This can take a lot of forms:
From a woman's perspective, this whole idea is completely ridiculous. There is nothing wrong with being gay, or effeminate, or wearing pink. But for one reason or another, even the slight hint that he might not be the biggest macho man on the planet is enough to wreck a guy's self-esteem for a month.
So, yes. It's silly, and they're being sensitive about nothing. But if you want to keep him happy, you'd better steer clear.