If a couple was childless and adopted a child, that child, when they become of age can choose to convert. The Rabbi’s issued this edict because they forsaw the problems of someone who may have converted just to marry a jew. Unfortunately, a sincere convert is a rarity, and to protect the rest of the community, the Rabbonim decided to issue this strict edict. But that doesn’t mean we are exempt from respecting a Ger/Georet. As a matter of fact, I have a close friend who is a Georet. She is always warmly welcomed into our home and loved by our children. Would I love her to marry into my Family, yes, but we are Kohanim anyway and according to the Torah, not allowed. How come no one takes issue with the Torah’s ruling in regards to a Kohain not being permitted to marry a Georet? She can be sincere and pure, but it is still not allowed.
Personally, I know of someone who did “convert” to marry a jew. They got divorced and she went back to her ways. The 2 kids they had identify themselves as Christian.