In the gay-marriage debate, one of the conservative arguments is that if you're going to change the law to allow to consenting, same-sex couples to marry, what's to stop you from OK'ing bestiality, incest, or polygamy? Not only is it offensive to liken gay couples with incestuous siblings, but it's just so absurd. Who would petition for the right to marry a sibling or family pet?

But maybe it's not that absurd. Just last month a German couple --- a brother and sister who grew up apart but now have fallen in love and produced four children --- filed suit to challenge the country's incest laws. Sexual politics are tricky, especially when they tackle taboo subject matter. Over his more-than-50-year career, American playwright Edward Albee has reveled in making audiences examine difficult, disturbing questions. In "The Goat, or Who Is Sylvia,"Albee explores some of the most cringe-inducing corners of sexual deviancy, but still manages to generate humor out of the most humorless subject matter around.

As Shipping Dock Theatre's production of "The Goat" opens, the audience is introduced to Martin, who is coming off an amazing week. He celebrated his 50th birthday, was awarded a prestigious architectural prize, and was chosen to head up a huge, billion-dollar project. His home life is seemingly equally idyllic, what with a smart, sassy wife who adores him and a bright teenage son who happens to be gay. His old prep-school buddy comes by to interview him for his TV news magazine and notices that Martin is deeply distracted. The two talk, and eventually Martin's secret comes out: for the past few months he has been having an affair. With... a goat.

As Martin explains, it's not just sexual (although that is part of it). He really, truly believes this goat, which he has named Sylvia, is in love with him, and vice versa. Martin's friend recoils in horror, but Martin remains fairly matter-of-fact about it. He's never had an affair in all his years of marriage, never even thought of it, but now he's in love with another woman. A goat.

Act II begins with Martin's wife, Stevie, opening a letter from the friend, giving her the bad news. You can imagine her reaction. The married couple spends the entire Act in one huge fight, which involves plenty of obscenities, innumerable broken dishes, and some hysterical wordplay. (Even in the midst of a blistering argument, Albee's characters take the time to correct one another if they misuse grammar: "It's ‘whom,' not ‘who,'" "crest of the hill, not top of the hill.") The couple's son, Billy, occasionally chimes in, and his accusations are met with some fairly venomous anti-gay remarks from Martin. Things get worse from there as concepts even more twisted than bestiality get dragged into the discussion.

On the night I attended, Benjamin K. Banks had just taken over for the role of Martin after the original actor backed out due to a family health emergency. (Banks will continue to play the role throughout the remainder of the run.) Given the last-minute switch, Banks read from script in-hand, which hampered the first act's rapid-fire dialogue.

However, Barbara Lobb's Stevie more than picked up the slack. Lobb commands the stage, especially in Act II, as she tries her damnedest to understand how the man she loves could love a barnyard creature in the same way he loves her. Her performance runs the gamut from shock to despair to rage to acceptance --- it's like watching the five stages of grief play out before your eyes. Credit also goes to director Mark Wenderlich's staging, in which Stevie and Martin circle each other like beasts on the hunt. (Or is it prey on the defensive?)

"The Goat"offers no easy answers, and the audience I watched it with seemed uncomfortable with the ending, at the very least. But if you're looking for a well-written, well-acted night at the theater that'll make you think, look no further.

The Goat, or Who Is Sylvia? | through March 25 | Shipping Dock Theatre, 31 Prince Street | $10-$22 | 232-2250, www.shippingdocktheatre.org.