Joseph, it should be further clarified that gentile women in the news.are not over on anything. However, it is not a marriage. A gentile woman who cheats on her friend is not over on anything.
As for polytheism, firstly there is a well-known machloket as to whether or not avoda zara b’shituf is prohibited to gentiles and secondly, not all Xtians believe in the Trinity. Unitarians, for example, do not and only consider that man to have been a great spiritual leader whose teachings are authoritative and perhaps a prophet.