<human>: Could you distill the essence of the text into 1-3 sentences?

Hey guys, new to the site and just needed advice from fresh, unbiased minds. My girlfriend of two months whom I really like was open about having hooked up with her male best friend a few times in the past. She insists that they're relationship is now purely platonic but I am still uneasy about their close friendship. I have met the guy and actually 
really like him. 

He can be really flirty but is like that with all women. However he also constantly cheats on his current girlfriend and I am uneasy about his close relationship with my girlfriend.

Would like to know whether I am just being insecure or if it is natural to be uncomfortable with their relationship. Thanks!
<bot>: female 24 with girlfriend 26 feeling uneasy about girlfriend's best male friend whom she hooked up with in the past. Need advice on situation.