I mean, c'mon, it's sex!  Fornication.  Coitus. 
The two-backed beast.  A little
somethin-somethin'.  We're genetically programmed
to like the stuff.  :)  It goes with being mammalian!

And no, I don't have any problem with the usual
themes of hentai, yaoi, or yuri.  Yes, a lot of
them are taboo; but in erotic fiction, taboo is
deliberately flaunted to create excitement.  The
key word here is "fiction," folks.  Most
non-hentai anime is chock-full of things we
wouldn't want to do in real life (wars,
assassinations, betrayals... extensive and
complicated love triangles... ;), but we enjoy it
anyway.  Often even because of those
elements.  It's the same for hentai.

As for the tentacle thing, that's an old Japanese
standard.  I've seen antique netsuke which depict
women... shall we say, "enjoying the company" of
octopi.  *grin*  Anime monsters are just the
latest incarnation.  File under "taboo," above,
and enjoy or don't, as you please.

But I don't want to hear anyone here telling me
that I'M not supposed to enjoy it,
particularly not for such a silly reason as
"you're a girl."  ;)