The dog of my aunt, I started off with, always came up to me, pushing his nose into my crotch, starting to push gently if he wanted sex. If I want sex, I have trained the dogs I played with so far on things others will not do accidentally, like rubbing his toy in my crotch, at a certain spot in my flat, since often I played with dogs from friends while taking care of them during e.g. holidays. Then of course, I do not want them to go back and behave rude or strange all of a sudden. Generally those dogs sit on those spots or repeatedly run around them in circles and make noises, if they want sex.