||Gemara Shabbat 13a
Ramba"m Issurey Biah Perek #21 Halachah #6
Tur & Shulchan Aruch Even HaEzer #21:7
Not much room for confusion.
They want a source why it is not allowed. Now you ask them for a source why it is allowed. They'll say, "people do it."
Great source! So we follow a religion of what the "people do." And if you want to differ from that, you better have a good source, which wouldn't help anyway, because "people do" is a much stronger source. Strange!?
But they wear a black hat and somehow God will just have to overlook some of the modifications made.