I see flirting at the gym the same way as flirting with your coworkers or flirting with classmates, it seems like a good idea at the time but you're left having to deal with them back at the gym/work/school no matter what happens. If you enjoy some place you want to make sure you have as few deterrents as possible. Plus it seems everyone I know started lifting to get girls, then realised that they really enjoyed the progress gained so lifted for THEMSELVES and the confidence they built up from that allowed them to hit on girls in bars.