I imagine it's difficult to get the measurements and reactions just right so that there are very few (hopefully no) false negatives and positives - but then even chemical contraception is not 100% effective. I can see this becoming a massive headache if it isn't tested enough. I also don't see people using this responsibly.
I could also see a ton of women not using this because they would be worried that it would offend their date.
I assume it'd rank somewhere between asking your date if s/he has a STD and asking if that spare tire around your date's stomach is for a truck or a van. In other words, Tuesday night for me.
Well here's a practical question: Who the heck carries around their own straw? Don't the drinking establishments usually provide that?
Some OCD people like carrying their own utensils and stuff around. You probably can't do so without looking like some sort of neurotic though.