Um, I'm probably alone on this one, but I don't even go on a single date unless they know.
I'm assuming we're talking about dating boys here, and that you're pre-op. (I think rules slightly change when you're post.) And also, I think "passability" is a very real variable here (unfortunately, but i think it's a variable that comes into play in all our lives in some way and in varying degrees). How "surprised" will he be if you told him? Rules change if you met him in a "liberal part of town" or through a "gay friend" than if you met him in a "straight bar in a conservative part of town" or a "religious anti same-sex marriage campaign."
That said, I've found boys to be weird creatures. They seem to feel easily "owed." What I mean by that is:
They buy you a drink, you owe them something.
They take you on a date and pay, you owe them something...etc
And when they feel "owed," pulling the rug out from under them makes them feel "taken" like you some how ripped them off. OK, if you've read this far you're at least trying to follow me, thanks. LoL Allow me to further articulate.
When they feel "owed" and you tell them, they tend to react in a less favorable way than if you were to be upfront. I don't think going on a number of dates affects whether or not they will be "OK" with it in the end. It does, however, give them an opportunity to react in a
bad way.
A man you tell, after dating, that is "OK" with it will tend to be more upset for "lying" for a short duration of time, than if you had originally been "honest." A man you tell, after dating, that is not "OK" with it may get violent/angry/physical/loud, when he may have just said "ew, no thanks" if you told him prior to dating.
I'd rather just tell and skim the negative boys off the top. I think it presents less danger. If a guy automatically doesn't want to "get to know me" after that, then so be it. I think it's better than hoping "love" or "chemistry" will aide a man into overlooking
that little detail.
These are my findings, anyway.