Dating is a stage of romantic relationships in humans whereby two people meet socially with the aim of each assessing the other's suitability as a prospective partner in an intimate relationship or marriage.
At a minimum, it is a waste of your time, as more likely than not, non-exclusive relationships peter out. It is rude and disrespectful to be subject to such behavior. Dating someone while they’re dating other people is the same thing.
Do you know how many monogamous blondes there are in the world? This suggests that all orientations can be physically aroused by depictions of different sex, but it does not mean that all women are bi.