When they’re in a relationship. Personally I find it disrespectful, but can obviously understand that a “like” has no meaning behind it. I can be rational about it, everyone has the right to like whatever they want etc.
Unfortunately for me my ex would do it and then cheat (or attempt to) with said bikini photo women so now I’m a paranoid mess.
Also a “like” to me indicates that someone is attracted to someone if the photo is provocative or almost nude. I have to remind myself people fire “likes” out left right and centre but it’s taking time to process this.
It’s hard because I wouldn’t like a topless mans picture and I expect that in return but never receive it. Why can’t you just like it in your head without clicking the button 
I’m told I’m making a big deal out of nothing because the females are friends.
For what it’s worth I don’t think I’m gonna be cheated on or betrayed again. I just don’t feel good enough and it fuels that fire.
Please be gentle-ish coz I have the self esteem of a lesser spotted sea slug and could just do with some pointers on how to not kick off and get over this. I’m not paranoid with other relationship things this seems to be my emotional trigger