I think a man having female Facebook friends is 100% okay, subject to the following caveats:
1. If one of the man's ex-partners was abusive, the girlfriend has a right to ask that the man not be friends with [man's abusive ex] on Facebook.
2. If a particular woman is the girlfriend's abusive ex, the girlfriend has a right to ask that the man not be friends with [girlfriend's abusive ex] on Facebook.
3. It's also okay to ask that the boyfriend not friend the girlfriend's family members on Facebook if she's not ok with that for whatever reason.
4. Exchanging flirty *private* messages on Messenger *may* constitute emotional infidelity, if one or both of the people messaging think there is a chance it could lead to actual physical intimacy. This isn't cut and dried, but it is an okay topic for discussion between partners.
5. Flirty public banter on Facebook that is clearly not serious should not be a problem. As an example, some of my straight male friends exchange flirty public banter with each other on Facebook - they clearly have no intention of progressing to physical intimacy, they're just expressing mutual admiration and/or good-natured teasing.
no subject
1. If one of the man's ex-partners was abusive, the girlfriend has a right to ask that the man not be friends with [man's abusive ex] on Facebook.
2. If a particular woman is the girlfriend's abusive ex, the girlfriend has a right to ask that the man not be friends with [girlfriend's abusive ex] on Facebook.
3. It's also okay to ask that the boyfriend not friend the girlfriend's family members on Facebook if she's not ok with that for whatever reason.
4. Exchanging flirty *private* messages on Messenger *may* constitute emotional infidelity, if one or both of the people messaging think there is a chance it could lead to actual physical intimacy. This isn't cut and dried, but it is an okay topic for discussion between partners.
5. Flirty public banter on Facebook that is clearly not serious should not be a problem. As an example, some of my straight male friends exchange flirty public banter with each other on Facebook - they clearly have no intention of progressing to physical intimacy, they're just expressing mutual admiration and/or good-natured teasing.