LGB (T is a separate thing) from a biological standpoint are variations of the reproductive instinct. The instinct itself evolutionarily came about a very long time ago and can be seen in pretty much all multicellular life.

Humans have evolved further as there is a need for a large parental investment for the offspring to survive, and have a mechanism that creates a neurological response based on the emotions of others that includes more complex interactions. For example feeling good after we have helped someone we care about (oxytocin and vasopressin associated dopamine response).

Therefore, for while not reproductively beneficial, LGB follows (or at least can follow) the rest of the patten of a healthy relationship. Circling back to the original topic, a mutually exclusive thing with pedophilia. Its a bit more complex than that, but that's the gist.