Seeing people whom have had sex with your SO

So just want to get other peoples opinions for when you see someone who has had sex with your SO. Or had relations with them? I know the past is the past, but when i seen her it was pretty damn awkward! I was just in a situation where a female that my SO slept with came to my job but i was thinking of her having sex with my man. And it kinda just makes me mad seeing her face .