Good point about the multiple orifices that won't cause a pregnancy. And, additionally, copulation in "side sex" can be done even on the chest, or on another flat surface of the body. "Insertion" isn't even a prerequisite (although it obviously can still be done non-vaginally or non-anally).

"Side sex" might seem weird or underwhelming to a lot of people. I think this goes back to insufficient sex education, and how local school boards can be so puritanical in terms of WHAT is taught to kids. Or, if parents are giving "the sex talk," they often center what worked for them or what they themselves experienced. This is particularly harmful for LGBT+ teens when it comes from heterosexual parents.

"The man may opt out of pregnancy by not impregnating the woman. The woman may opt out of pregnancy by terminating it, and once the baby arrives, they are both on the hook unless they can get someone else to adopt it. If a man wants to opt out of parenthood after the baby arrives, he can do so, provided he can convince someone else to legally assume the co-parenting responsibility by adopting the child."

But isn't that what adoption agencies are for? And you still have the hypothetical scenario where one biological parent wants to raise the child, but the other parent wants absolutely nothing to do with the kid.

I realize that people will claim that an "opt-out" window puts undue pressure/burden on the parent who wishes to keep/raise the child on their own. But then, that goes back to "You should have thought of that before you participated in making the baby!" -- which is essentially what you're already saying to the male parent, in terms of what his post-coital responsibility/onus should be.