This is called an "ejaculatory anhedonia" and is likely a result of taking SSRI's.

As HT advised, please seek medical care for it - this problem is unlikely to go away by itself.

Wishing you luck finding a solution. I do not want to sound alarmist, but I do want to communicate a sense of urgency: you need to seek medical care ASAP, because sometimes these problems become "tardive" - even when the offending medication is removed, the horrible side effect remains.
http://en.wikipedia.org/wiki/Post-SS...l_dysfunction:

"While apparently uncommon, it can last for months, years, or sometimes indefinitely after the discontinuation of SSRIs"

In general, with all side effects that have the potential of becoming "tardive", the longer you take the medication, the more there is a chance of a "tardive" side effect. ==> call your doctor on Monday and raise your concern.

If you are not on any medication, and have never been on one, then I do not know what it might be, but at any rate, prompt medical attention is the order of the day.