That's not quite what it is. She (the person who sent the letter) said that as an off-detail, she was asking how they might grow a family without physical connection, which is entirely possible. You don't need that in your family if you choose someone likeminded to you. Of course, though, Peterson doesn't read the whole letter, it's normal to do highlights with those.

There's also a bit of context. Sexuality is a large part of Peterson's teachings, though he takes an exclusively popular political approach to it. To give the most relevant example, in one of his most famous teachings, he put forward the notion that incels are a result of society casting them aside, with this in turn owing its context to the incel movement, which consists of people expressing angst over disproportionate relationship statistics. He has said this a few times, it arises out of ideals stemming from the sexual revolution and culminates today in seizing the means of reproduction, to use a double entendre. For this reason, incels and asexuals are diametrically adjacent on the "who do I please" spectrum.

So then this guy comes along, who many wonder if he's there just there to please, and then this woman asks the question in the letter. She never says "how do I be physical", she's asking "how do I find peace", and with the context in mind, his mindset becomes a real awakening (not in a good way). Unlike being an incel, asexuality is not only a way we're born as opposed to just some random identity, but it's also the natural state of things, to have a drive is not necessary for life to exist. Side note, adoption is never mentioned.