I have been looking some interesting writing of Rav Jacob Emden. It is true that he allows a girlfriend type of relationship but not exactly in the way you might think, Rather the idea is that he comes and lives with her man in his home. It is not to be casual. [see Chronicles I chapter 2 verses from 45 46 and onward. He thinks that there is good reason for a zava to need to be tovel in a natural spring as Rashi and some geonim say. I might mention that this is not stated in Leviticus. In Leviticus a natural spring is only mentioned with a zav. However there is a hekish between zav and zava in the Gemara that would seem to mean that a zava needs a natural spring. If you count like the Rambam all women will be zavot. [No one sees blood exactly from day 19 to 25 every single month. But if you count like the Ramban, almost no women will ever be zavot. They do not usually see seven days and then again three more ] And where could you find a natural spring anyway? The sea does not count as a natural spring