Most relationships I have ever been in began with flirting or the use of sexual innuendo. Flirting is a game which allows participants to "test the waters".
If there is mutual interest both parties raise the stakes and gradually take things further. If one person is offended the other can always fall back on the old line of "I can't believe you thought I was serious!" ha ha ha
I think people will always want or need to feel they are attractive/sexy to people other than their mates.
The people who believe flirting is "innocent fun" are often the same ones who after being caught in an affair will tell their loved ones, "We never planned for this happen" or "One thing led to another." A teacher I once had said:
"The kiss is the persuasion to lower invasion"
Flirting is verbal foreplay. :-)
No one wants to think about or let alone see their spouse/significant other flirting with someone else!