I think we do, but it's a strange idea unless we think of it as passing rules, quasi laws, for ourselves, and that may go against the tenor of what it means for us (no joke intended, I"m monogamous and good at that): it seems more like lying, and it's not obvious that we define 'lying', only make room for speech that may be false but isn't meant to deceive.
If I tell my sexual partner they are e.g. not allowed to act on sexual fantasies, for others, in any way, as opposed to not consenting to sex with others, and so on ('don't fall in love'), then I am in some sense giving a stricter definition of infidelity, but is my partner engaging more/less in infidelity if they act that way?
It's that strange interface between personal/private lives and the publicness of ethics.