Giving men power of their own birth control does not lessen the power women have, except in the case of coerced trust (i.e., "trust that I have birth control since it's now possible, even though I really don't"), or in the case of wanting the power to deny contraception, which I think many would agree seems unethical, given the legal responsibilities men have to their offspring.
It doesn't lessen the power women have, it just gives the same power to men. Specifically we're talking about the power to decide whether to risk conception (assuming it's not mutually desired) or not in advance to the sexual encounter.
Yes, lying about contraception in order to trick a man into getting you pregnant is unethical, but it's not unheard of. Women are humans (duh), some humans do unethical things, some of those humans are women, using pregnancy as leverage.
Personally I wouldn't think this behaviour is very widespread (although I have heard women joke about it being an option -- not implying any of them would actually consider it) but considering the significance of the consequences, I think it's understandable why some men might value this risk far greater than its actual prevalence would suggest, regardless of whether this risk assessment is rational or not. The tired meme of the poisoned M&M springs to mind.
