What difference does it make? They're both equally expressions of strong emotion; whether you find one more acceptable than the other is purely a matter of personal upbringing and perception. Like I said before, different cultures and generations have wildly different sensibilities about the relative intensity or shock factor of different expletives. It's all completely arbitrary, so it's meaningless to worry about the distinction between one curse word and another. Grammatically, emotionally, they serve interchangeable purposes.
I mean, really, "bastard" is a problematical insult if you think about it. It means an illegitimate child, and the idea that there's something intrinsically unworthy about a person born out of wedlock, that a child should be blamed or hated for a transgression committed by their parents, is an antiquated, ugly, and cruel notion. If anything, it's conceptually a much more disturbing word than the mere sex-act allusion you suggest as a substitute. Like I said, this is all arbitrary and it doesn't hold up if you really think about the meanings and contexts of the words rather than just your immediate kneejerk reaction to them.
