Well, without going into over-interpretation (which may merely be projection, in certain cases), it could be about not being able to get a hard-on in the first place, and being under pressure, as you say. Or it could be about getting it, and losing it, getting it, losing it, at certain key moments (which — I'm told by a friend — happens…).
Either way, it's always seemed to me that that's the reference, in a suitably discreet way, because this had to played on the airwaves nationwide (and, as I remember it in the summer of 1970, was. It was all over the place). Of course, it's framed very romantically. But I'd be astonished if I were the only person who understood it that way.
Now Kylie's a frisky girl, and I'm pretty bloody sure she got it, loud and clear, but then she thought, “Well, this works for girls as well — not obviously, not being able to get a stiffy — so why shouldn't I sing it?” Love her version too, incidentally. That trill, which she does twice — now that really does it for me!
And if anybody thinks I'm talking too much about sex. Well… rock and roll, blues, soul: a lot of it does boil down to sex. When Robert Plant sang about “I wanna give you every inch of my love” does anyone think he was speaking purely metaphorically? Which made it interesting when Marsha Hunt decided to do a version of that. But then, you think for a second — that works for girls too.
Shake your moneymaker!