Maneater

I read Three Times a Lady and it sounds like the Morrigan can be just as much of a hard assed rapist as any male god. She can also switch between ugly and ravishingly beautiful, so why does she need to force such lusty gods like the dagda to have sex with her? Is her reputation really that fearsome?

In a word, yes. The Morrigan is goddamned terrifying. She's not a very nice person and all the Tuatha tend to know it. I'm not going to repeat any of the vile colloquial phrases John is saying behind me, but there's a certain level of insanity that starts to make dudes feel uncomfortable about having conjugal relations with a lady, and the Morrigan meets it. I'd also keep in mind that while by Scion's rules she probably has Visage Great and Terrible and can swap from scary to beautiful at a moment's notice, the Morrigan is almost always described as terrible to behold (though I would note that the Dagda seems to think her backside is quite fetching when he happens upon her bending over the river).

If I can get outside of rules for a second, though, I think some of the reluctance probably comes from a more subtle idea: when the Morrigan is demanding sex, that's kind of a scary reversal of power for a culture in which the male warrior-king archetype is supposed to be the one in charge. A lot of mythologies have a certain distrust of strong female sexuality and tend to feature male figures freaking out about it, such as when Cu Chulainn refuses the Morrigan or Zeus has to get somebody to marry Aphrodite just to get her sexuality under control so the whole pantheon doesn't blow up. There are a lot of psychological theories about why this is, ranging from fear of the mysterious power of females to reproduce to equation of the initiator of sex with the person with the power to control the situation to fear of the male's power (symbolized by semen) being outright stolen by the female. Even if you don't particularly believe in or ascribe to any of those theories, there's definitely a strong feeling across many mythologies that women who demand sexual favors are scary as shit.

In essence, there's a lot of men raping women going on in ancient myth, but when women start doing the same in return (and the Morrigan's not the only one; Ishtar and Aphrodite are also pretty fearsome when they decide they want a dude), people start wigging out. It's just not something that makes sense for most ancient cultures; a man might rape a woman and that would be, if not necessarily socially acceptable, an understandable part of the natural order of things. A woman raping a man, on the other hand - that's some scary, unnatural madness when you're living in first-century Ireland.