The Superman/Lois problem is addressed in Larry Niven's essay "Man of Steel, Woman of Kleenex", questioning how the blood line of the House of El could be carried on. The problems are more or less fourfold.
1:
Superman is really, really strong and can easily break anything if not careful. He would have to have control during sex approximately equivalent to a human handling a cellophane baggie full of Jell-O.
2:
Each individual cell of his body is also really, really strong. This is handy when, say, fighting off diseases or microscopic supervillains, but causes trouble when thousands of sperm cells all want to impregnate the same non-invulnerable egg. Pow.
3:
Kryptonian infants? Also super. If fertilization is managed, one kick in the womb and we have an Aliens-esque situation.
4:
Superman may not swing that way, but this is only speculation and is not addressed in the essay.
Kryptonite is one possible solution to problems 1-3, but somewhat awkward and painful. A more useful notion may be to hold the entire affair on a planet orbiting a red sun, where Kryptonians have no superpowers. The other option is for him to have an equally powerful partner. Wonder Woman is a top candidate; Supergirl would be except that she has a 66% chance (depending on who writes that issue) of being his cousin. Magic would also work, but feels like a bit of a cop-out. With regard to #4, there is not much to be done, although it does ease the problem of a sex partner's survival somewhat. (There are many more invulnerable males in the DC Universe than invulnerable females.)
The Flash, when in motion, generates a force field that nullifies friction but allows traction (somehow). He would probably not set his partner on fire but I can't imagine it would be all that great.