Since both of them were underage when they lost their virginity, I can’t really make art about it DX However, I can still tell you!

When Venus was a homeless filly, some stallion lured her to a shack with candy and… ya know. She was kept there for awhile being their pet until a curious colt Mars found her and got her out one night. 

Mars lost his virginity when he hit puberty and got some weird urges which he took out on Venus out of nowhere.