I don't understand the statement. Does "faithful" there refer to romantic relationship "fidelity"? And what are the options in this case? Does it mean that if they have good options for people to cheat with, they will cheat? If so, no, that's a cynical and just wrong statement. Many people wouldn't cheat. Or does it mean somebody wouldn't stay with their partner if somebody better comes along? That's more true, I guess, but still not even close to being categorically true. And, btw, I think it's generally more true for females than for males. Females see their men as interchangeable appendages, and they will "upgrade" to a better model with a better checklist of generic qualities if given the chance.