I'm going to tell my gf she is being sexually selfish
Why do you feel that way about her?
I know she can have sex whenever she wants to, but for me it's ok only two times a week.
You shouldn't verbally attack your partner just because you want different things. You should respect her desires, but let her know when she asks too much.
What's wrong with telling her the cold hard truth?
If you simply accuse her of being selfish, at best, you will just make her feel bad and possibly drive her away.  You should let her know that you love her, but that you only have a certain amount of physical stamina.  She will surely understand.