First, this answer depends substantially on what the nature of your relationship and break-up was like. Did you really love this man? Did you see yourself spending a lifetime with him? Or was he a Mr. Right Now? Who broke up with whom? Those questions will all effect the outcome of rebound sex.
Personally, I never see rebound sex as a bad thing. I think it is pretty much natural; there will always be that person that we sleep with after the end of a break-up. Rebound sex can be very cathartic, and can help us move on. Having sex with someone else, especially after a long-term, monogamous relationship ends, helps to create distance between the memory of you and your ex’s sexual relationship. I also know that it can hurt like hell to have to actualize the reality that you are starting all over again. But that's what rebound sex is all about: a rebound.
Know that this will not be your last endeavor into the playing field, but more of a jumping off point from which to start. It can add to the grief you are already feeling, but it's often in a way that will be necessary to further move on from a relationship that does not exist anymore. I would recommend for your first time back out there to call on someone you've slept with before, someone who will understand the nature of rebound sex and handle it with the care needed. I found it particularly useful to inform my rebound sex partner of why I was there. He knew it wasn’t going anywhere after the encounter, and it seemed as though he gave extra effort to make sure I enjoyed myself.
Another reason I like rebound sex is because it can also be very empowering and a great way to get back in the saddle as a single person. Rebound sex can remind you that yes, you are single. There are plenty of attractive fish in the sea. You are attractive and desirable. I don’t say this as though anyone should seek or need external validation. This is already a difficult time and sometimes we need that reminder from somewhere/someone else that we are sexy, dammit, and your ex is not the last person on earth to think so.
I do want to encourage you to explore what may be holding you back from moving on. A few months really isn’t that long, depending on the length of the relationship. Having sex with someone new brings about the sense of finality to the relationship that is now over in a very real way. It could be that you are not fully ready for that finality. Perhaps you have held onto the idea that maybe the relationship isn’t really over, that maybe you two will get back together. Sex with someone new puts an end to that thought process. If you are a person who attaches emotionally to sexual partners, maybe you aren't emotionally ready to risk being hurt again.
I think rebound ex is worth considering. My personal opinion has always been that the best way to get over one man is to get under another.