Some instances of statutory rape are entirely unforced by any common definition of 'force'.
(The law doesn't see it that way, but the law also sees statutory rape as a strict liability offense, meaning that lack of intent to commit it isn't a possible defense. In short, if someone lies about their age to have sex, they just got raped.)
IANAL, but I believe the reasons it is a crime are the same: in the off-limits situations, the minor is believed to not being able of giving informed consent. But at least it's not called "rape".
(I find the term "Statutory rape" quite misleading. It is definitely not the same thing as plain rape, or worse, child rape. It looks like it has been pushed by a lobby of afraid, protective, conservative, possibly religious parents.)
One of the saddest things I can remember (though, unfortunately, the details escape me due to time) was a couple of rape cases that occurred in the US maybe a decade or so ago. One was a case of a very violent rape where the sentence was something on the order of 15 years. The other was a case of statutory with a fully consenting 16- or 17-year old where the sentence was something closer to 40 years. I was pretty young at the time (if it was a decade exactly I would have been 12), but that's definitely when I first realised that the "won't somebody please think of the children" arguments are bullshit.