Would you be more upset if your SO ended your long-term/serious relationship if he/she was cheating with someone of the preferred sex or if he/she realized he/she is gay/straight and is cheating on you with someone of that particular preference? As in, if you're a straight female, would you be more upset if your boyfriend left you for another woman or if he left you for another man? Change the variables as applied to you.
Why would you feel this way?
Has this ever happened to you or someone you know? How were you/they affected by it?