Unfortunately, this is a really common side effect of SSRIs (which Celexa is to my knowledge?) I'm going to get a little sciencey here, so bear with me. SSRI's (selective serotonin re-uptake inhibitors) work by controlling the absorption of neurotransmitters in your brain. Basically, your neuron fires, spits out a bunch of neurotransmitters (in this case, serotonin) and the synapse then absorbs that neurotransmitter. Whatever isn't immediately absorbed gets taken back in (re-uptake) by the neuron that spit it out. People with depression have a problem where the neuron re-uptakes EVERYTHING before it's had a chance to be absorbed by the receiving neuron, so you basically never have enough of it. SSRIs come in by blocking then neuron from sucking it back up immediately and giving the receiving neuron more time to absorb it. This is great for depression!

Bad news: Orgasm is a huge, huge flood of neurotransmitters that are almost immediately reabsorbed (normally.) That's why they feel so good: a huge, absolute FLOOD of chemicals in your brain. But you can get up and walk almost right away; the afterglow doesn't last long because the reuptake is so fast. But since an SSRI by design blocks that flood from happening to start with? You get all wound up and end up getting no where.

The only SSRI known for NOT having this is Welbutrin, but that one's a bit dodgy for some people because it has other side effects instead. I've taken pretty much every SSRI there is and always had this problem, but I got switch to SNRIs instead (I'm bipolar) and haven't had the issue anymore. So I take Remeron and it's worked.

Talk to your doctor about it, because like I said, it's a really common side effect that they consider serious, so he or she might be able to suggest either medication to counter that effect, or a different SSRI altogether.