With their mouth??? They're not mouth breeders, and even they don't really do it that way.
Male guppies have gonopodium [sp?] The bottom fin in front of the tail has a tube in it, unlike the female's lower fin which is pretty flimsy. And yes, that's what it's used for You will see them flip that little thing forward as they approach the female attempting to mate.
Yep pondlady is right. I've seen the male trying to coax the female to accepting him but it doesn't always work It's properly so quick that if you blink that you will miss it
Is it true that male guppies perform an 'S' dance in front of the female to try and impress her before mating comenses?
You will often see male guppies swimming very close behind pregnant females. All they are waiting for is for the pregnant females is drop the live fry and they will eat them as they pop out. This may be what you have seen