The miata is girly because it doesn't have enough power to get into manly shenanigans (burnouts, slides, etc). More power makes anything more manly, even a VW beetle. I'd probably drive either mazda if it were given to me, but I'm not too impressed with their interiors.
I'd happily drive either Porsche. Yeah, the boxsters are a little girly (because they're cute like a miata) but they sound sexy and I hear they are wonderful to drive. The cayman is more manly because it has more power and it's a hardtop. Women will always be more likely to drive convertibles all else equal, because they like the ability to have their hair blowing in the wind (even if it would mess up their hair). I'd venture it has to do with them being more glamorous, and since most women are naturally insecure and have self-esteem issues they want to be more glamorous to feel better about themselves.
Hardtops are rigid and unforgiving like the will of soviet Russia!