Is it Okay to Flirt with Someone You Know is in a Committed Relationship?

Dear blog readers

please I would love to read what my fellow blog readers think about this. Talking about this with my friends yesterday turned into a very big argument.  

If you know someone is already in a relationship but you don't personally know who their boyfriend or girlfriend is, you just know very well that this person is in a committed relationship with somebody ... do you have any obligation to respect that person's relationship and not flirt with them?   

Or do you think it's completely on the girl or guy and you're free to do whatever they allow you to do with them? Especially considering both of you have made no vows or engagement proposals to anyone and this person is willingly making himself/herself available to you?