I know what a strap-on is, but what kind of insult would it be to say that somebody “has a strap on”? Does it mean that they’re gay? That they’re a wanker?
If the insultee were male wouldn’t it suggest that they had been emasculated?
Was this spoken by a Brit (or ex-colonial)? If so, it may have been “have a strop on” (look under “S”). It means that he is in a bad mood.
hahaha 
Peter Phile. I am looking for a Peter Phile.
If said to a guy, I guess it means he has to use a strap on to satisfy a woman because he either lacks a penis, or it’s too small to be of any use.
Or, his wife/GF uses a strap-on on him, because obviously any guy who likes anal play is gay, and being called gay is the worst insult any straight man could ever get.
If you say it of a man, you are impugning his masculinity.
If you say if of a woman, you are impugning her femininity.
In the “Billary” era of the Clintons I used to always joke that in the Clinton marriage Bill wore the pants, but Hillary wore the strap-on.
If a man isn’t acting courageous enough, you can tell him to “strap it on” and get to it.