I'm in this situation right now. I will try to pass this with some therapy and exercises, because the medication decreases libido and I don't want to lose my girlfriend.
That said, I'm going to make the brash counter-cultural statement that if you wait until after you commit to each other for life before having sex, then you worry a lot less about whether you are going to lose your partner over sex. If a necessary medication causing a libido blip is enough to derail your relationship, maybe you should consider pushing sex further up the commitment ladder?
Couples I know have had trouble due to depression, childbirth, work-related travel, injury, and a year-long coma. While sex is a valid and important part of the relationship, it's also important to be able to weather periods of difficulty.
I should also note, depression tends to make one less capable of meeting other important relationship needs. If treatment causes a temporary drop in libido but improves those other areas, that should be a net positive. Thus, I submit that if a relationship gets derailed by such treatment, there were deeper issues in play.
The comment you were responding to said something considerably more nuanced and interesting than "lol" in conjunction with the "don't have sex until you're married" part. It's OK if you disagree, but please do so in a more thoughtful way.