Do you require sex to live?
Does the amount of 'pasty' you smash somehow equate to your perceived self-worth?
It can take a strong will to say no to sex when it is handed to you on a silver platter, but when it involves an ex-lover, and you were the one who broke up with her, you would be conveying weakness to her. By saying yes to her, you are effectively saying I can't do any better than you right now. But you must understand, being alone, is better off than going back to an ex.
Don't risk any old feelings coming back, otherwise you may end up in the ex trap:
- break up
- get back together for sex
- restart relationship
- repeat (except with each repetition, you increase the chance of children/financial/logistical complications)