I have a slightly different take. Straight women in open relationships have an easier time finding men willing to fuck and/or date them; their straight male counterparts have a much more difficult time. Stigma and double standards are at work here—she's sexually adventurous; he's a cheating bastard—and waiting to disclose the fact that you're poly (or kinky or HIV-positive or a cammer) is a reaction to/work-around for that. It's also a violation of poly best practices, like Veaux says, but the stigma is a violation, too. Waiting to disclose your partner, kink, HIV status, etc., can prompt the other person to weigh their assumptions and prejudices about poly/kinky/poz people against the living, breathing person they've come to know. Still, disclosure needs to come early—within a date or two, certainly before anyone gets fucked—so the other person can bail if poly/kinky/poz is a deal breaker.