Yikes. That's definitely a tough spot to be in. You have my sympathies, and big hugs if you want them.
It sounds to me like your ex may have done the "typical" rebound thing of rushing into sex and/or a relationship with someone else long before he was ready. That's sadly common -- people feel hurt by the ending of one relationship, so they run out and find somebody, anybody in an attempt to make themselves feel better. My ex did exactly that -- not a month after I left him, he was in bed with another woman, who he promptly dumped a few weeks later.
Be good to yourself, okay, hon? Ending a relationship is hard enough; watching your ex go off on a rebound right away is just adding to the stress. Do you have some friends you could talk to about this, or something fun you could do to make yourself feel better? I usually find that a new haircut (nothing drastic, though!) and a manicure always makes me cheer up for a bit -- but then, I'm such a femme at times.
"The only unnatural sexual act is that which you cannot perform." - Alfred Kinsey