The side effects of a medication vary widely and depend on the person taking them. Antidepressants are generally known, as a group, to contribute to a decreased sex drive, though there are cases of idiosyncratic effects where people experience the opposite. Furthermore, if interest in sexual activity was decreased due to a depressed mood, lack of energy and other symptoms, alleviation of these symptoms may lead to increase desire for sexual activity.
A friend of mine was taking two medications; a commonly prescribed antidepressant and an adjunct, which is typically marketed as an antipsychotic, but has off-label uses for boosting the antidepressant action of other drugs. He felt, in his words, "pretty much asexual", though he meant it in terms of libido and interest rather than a change in sexual orientation.
In my own case, I have felt mildly depressed in the past, and I wanted to avoid commercial antidepressants since it wasn't terribly severe. I have used St. John's Wort extensively in the past, and it helped my mood to a degree (I realize doing this without the supervision of a physician was not the best of ideas). What I didn't notice until I stopped taking it (just a month ago) was that my interest in sexual activity seemed to diminish, lower and lower until I had almost no interest at all. That's just a personal experience, though, and it was with an herbal supplement rather than a pharmaceutical antidepressant.