Your question is very good, and you get an answer that is fiction. Jesus never parses OT law, saying to follow this but not that. There is never a message to Christians to “follow OT laws as best you can,” especially sexual ones.
The ‘Christian law’ that results is far more extreme in controlling sex than actual biblical law, where divorce, harems, prostitution, etc., are fine.
Christians get cognitive dissonance when references like Hebrews 8:13 are pointed out. (“By calling this covenant new, he has made the first one obsolete; and what is obsolete and outdated will soon disappear.”) That Jesus, in Luke 16:16, says the Law ended with John the Baptist is ignored. Paul’s insistence that even circumcision, the basic covenant identifier, is not a Christian calling, is ignored. Christians circumcise their children out of a vague feeling it’s sort of Jewish and basically good.
So what we get is an idea of Christianity as a variant of rabbinic Judaism, in accepting OT rules as God’s ideal program for human life among the covenant people. The difference is that Christianity dispenses with all laws except those it reads as sexual. Thus it imagines a God who is focused intently on restricting human touching as the covenant identifier.
Without wishing to disparage rabbinic Judaism, my reading of the OT/NT situation is that the Laws are not an ideal program for human life, but rather a practical and provisional strategy for managing divine energy—in a world understood as having multiple temples to multiple deities.
The OT Law makes no sense away from the Temple. Following it without a Temple is not possible. And Jesus’ body becomes the Temple for NT believers, a conceptual shift that Christian tradition can’t make.
It hates the body! Christianity then becomes a spiritual system oriented around body-hatred, imagining controls of the body as the basic good.