The other thing is that people who hold these beliefs do not discard the OT. The homosexuality and crossdressing admonitions (among other things) in the OT get brought up a lot by evangelicals nowadays, and if you actually attend church and get into Bible study pretty rigorously, even if the Law is "perfected" in Christ, it's not considered to have changed.
We don't put gay people to death nowadays, for example, in the USA... but it's still considered an "abomination" by conservatives. (In recent news, the Lutheran Church is currently undergoing another schism because homosexuals in committed monogamous relationships have been permitted to be clergy; in the past, they were required to be celibate. Why's it a big issue? Because of the OT law, plus a few comments made by Paul.) It seems to be that the OT law and NT are still totally connected and referenced, but we just are a "kindler gentler nation" because we don't actually punish people for their specific "sins" in the prescribed OT way. That seems to be the difference, but the laws remain, in practice.
It's sort of disingenous to say that Christianity's all about the NT, because in practice it is not, and if you attend any sort of conservative church, you'll see that the OT is referred quite a lot in order to connect it to the NT and the old laws are merely being fulfilled in Christ rather than tossed.