I have been watching a couple of seemingly unrelated online conversations, and found a common thread: an assumption that seems to run through them all - an assumption that I don't share.
The assumption seems to be, more or less, that consent is a binary situation; that people either agree to something, or they object to it; that the absence of consent is the only thing that makes an arrangement unjust; and that when all parties agree to something, the agreement is thereby rendered just, moral, and ethical.
I would say, on the contrary, that consent is only one element of justice (and a sometimes-dispensable one at that); that consent furthermore is not binary, but in fact runs along a spectrum from vigorous objection, through grudging acceptance, indifference, and favorable approval, all the way on to enthusiastic agreement; and that consent is not simply a matter of free agents making deals on neutral ground in a free marketplace, but is actually entirely dependent on context, relationships, and leverage in negotiations. (Theodore Seeber illustrates the role of leverage in negotiations in his post on leonine contracts.)
Both conservatives and liberals get this, if you engage them on the right topic. Conservatives generally understand it as it relates to sex, and liberals understand it as it relates to commerce. A conservative would agree that two married adults who meet and have an affair are committing an injustice against their families, and that a man who "picks up" a recently-dumped woman in a bar and takes her home for a "consensual" one night stand is exploiting her emotional vulnerability and intoxication. A liberal would agree that desperately poor people who pick lettuce or flip burgers for subsistence wages may be "consenting" to the arrangement, but that doesn't mean their employers aren't exploiting them.
The key here, for people who are accustomed to fetishize the Commercial economic relationship to the exclusion of all others, is to realize that Commerce bleeds over into War on one side (and to the extent that it does, leverage becomes increasingly important in commercial interactions) and into Friendship on the other (and to the extent that it does, mutual loyalty and obligation become increasingly important.)