Sometimes it's unavoidable, but it's a concern that's worthy of consideration, and saying shit like "just don't have sex until you're married lol" isn't helpful.
The way you choose to build a relationship has a lot to do with how well you are able to sustain it through certain types of difficulty.
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.
What I found interesting wasn't so much what was said as the unstated assumptions behind it. The comment was in any case almost deliberately useless in how to handle an existing relationship that already entailed sex.
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.