I seem to be in the minority on this one, and fair enough- everyone is entitled to there opinion.
Just for me though- if I'm on a date with a girl and I'm dating dating another girl (as in still single) I wouldn't be telling the other girl about it. I know girls like pre selection blah blah blah but to me- it would just look like I'm trying to play someone.

Unless it's been made clear it's never guna go further than sex- and in that case it's totally understandable saying you are on a date. It could be kinda hurtful to the other person if he/she is starting to have feelings for the other person.

Well Gizmo, it depends on the perspective you're coming from. If you value honesty a lot, then it's not crazy to INFORM her that, at the moment, you're seeing someone else.

But to me, this ^ reason is the only reason to tell that to a girl.

It's so try-hard and show off if you're telling that to get her jealous. And I'm even wondering if girls won't see right through it.

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Ambitious to be succesfull => Shyler

You have got me confused. I said I wouldn't be saying anything so what's try hard (I may have misunderstood you)

I have this girl coming over tonight, she is 100% aware that "we" won't go anywhere.
On Sunday I have a date with a new girl. I'm not guna tell tonight's girl about my date as she doesn't need to know. Do you think that's wrong? I don't feel I have mis led her as she knows we arnt going anywhere.

I think you understood me wrong or that I had to make myself more clear. It's perfectly fine to just shut up about it. I would do the same.
I just meant that, there are people who value honesty a lot or even see honesty as a religion. For those people who want to stay honest at all times, I can get why they would say that.
I value honesty a lot as well, but I know there are times where it's better to just not say things because it might hurt or because it wouldn't matter or make sense.

But if you would say to a girl that you're seeing others not from a perspective that you want to be honest to her, then you would probably tell her to make her jealous or to impress her and I think that's just lame.

_________________
You WANT to make a change.
You CAN make a change.
You WILL make a change.

Ambitious to be succesfull => Shyler

I see what you mean and I agree with you. However I do think that most of the time if a girl says that it's coming from the jealousy point if view.

I asked that girl last night what she thought and she seemed to think that she did it to make my friend jelous. Maybe she didn't but it certainly looks that way.

What about just NOT responding? Seems like the best way to be unreactive, just don't do anything. Sometimes I get a weird text and I just don't say anything back. Going radio silent makes the other person second-guess themselves. Suddenly they're doubling over to appease you sending you little messages trying to make up for some perceived insult.

Nothing wrong with doing that. I have done that before- works great