my wife has been cheated on in two previous relationship because she gave those guys the freedom to hang out with other chicks. there was a point a few years ago in our relationship where she found out that i would sometimes go out for a drink with a female coworker on friday afternoons. i never told her about going out with the girl from work, which to her meant that i was trying to hide something and i guess i was. your girlfriend/wife/whoever doesnt like hearing about your female friends that she doesnt know, and thats why i didnt tell her. i never had any feelings toward this woman and i never felt like she had any for me, we simply got along well and felt like getting a drink at the bar next door once in a while. i didn't feel like what i was doing was worthy of losing my wifes trust, but hiding it from her was. i stopped going out with the girl from work because drinks with her wasnt worth giving my wife a reason to think i might cheat. i trust and respect her and i want the same kind of trust and respect.
on the other side of things, she had this long time guy friend who i had met and hung out with a few times when i first met my wife. they never dated, but she said that they did have sex once. referred to him as more of a "best friend" guy, but they stopped hanging out a while before we started dating. this guy i guess he was going through a tough time and texted my wife about how he missed her and wanted to hang out. she told me about it and told him the only way she wanted to hang out with him was if it was at our place and if i was there too. i believe she did this because she knew how i would feel if she told me she was going to see him someplace without me. he never came over and stopped contacting her after that.
thats my honest experience with this shit, take it how you want it. i agree with SFB and everyone else has said. nothing good will come of your girlfriend spending one on one time with some dude she has a crush on.