Liberals make a point of denying the importance and gravity of traditional morality. It is part of their secular humanist anti-Christian agenda. Violating the marriage oath is a rite of passage for them.
Also, they think it provides cover for their many promiscuous leaders to pretend that sexual immorality is no big deal. Clinton, Edwards, Barney Frank- they used to say “live boy or dead girl”, but Franks and Fat Teddy put paid to even that.