Without getting graphic,or vulgar,the gonopodium is the male sex organ (of sorts). The anal fin (immediately behind the anus) is shaped like a "male member"...and used for that purpose.
Guppy "mating" is really nothing more than him swimming up,pointing the gonopodium and trying to make contact with her vent...blink and miss it. Males are pretty persistent so it's an activity that you see briefly at least everytime you look at the tank.
Not too many species of "community" fish can interfere with this activity...but the desire to breed long-finned varieties of Guppies has made some males just too slow to make contact. Did I mention that females seldom sit still for this ?
Some "racey-type" fish,like Danios for example,can interfere just by their constant motion and curiosity to things going on.
Got a pic of the male ?
Just think about it.