Okay, I admit it. I didn't really pay any attention at all in my high school biology class, and I don't really have a very clear idea about how cells reproduce. I'm pretty sure they just split in half and then go about their separate business- kind of like skipping right to the divorce stage without even dating first- but I don't think that counts as having sex.
But is that what all microbes do, or only some of them? I honestly don't know. Far all I currently understand of the situation, it's entirely possible that some microbe of some type is actually having sex of some kind in my drinking water, and honestly I wish they would just stop.
So I looked it up, but apparently you have to have paid at least a minimum amount of attention in that old high school biology class to even be in a position to understand the explanation when you finally do manage to track it down. Apparently the chromosomes go from being diploid to haploid and then the gametes get together and make a zygote, which is also diploid. I did not actually find this explanation to be terribly helpful, as I have no idea what any of those words mean in the first place.
But, maybe you do, and will now be in a position to answer the question to your own satisfaction and either drink right up or refrain from drinking forevermore in a spirit of puritanical revulsion. That's what I intend to do.