Milkshake's comment about synthetic chemist intermarriage reminded me about a recent post by Liberal Arts Chemist on "research incest." Go ahead and head over there to read the story. Short version: a young PI catches his postdoc and his grad student in flagrante delicto; they are both married, but not to one another. LAC's question: is policing this behavior the responsibility of the PI? In the comments, LAC also notes that we're typically uncomfortable with directly addressing the infidelity issue.
If I were the PI, I'd want it out of my lab. Other than that, I don't feel that I have the power or influence to address it. I could imagine the HR- and E&HS-like bloodless speech that I would probably give: "Inappropriate place for such activities... lots of reactive chemicals around, bad chemical hygiene... affects the morale of the group, blah, blah, blah, etc."
Of course, I do have a moral and/or religious point of view on the issue. [Short, boring version: Marital infidelity is wrong, from a relationship and a spiritual perspective. Exciting, I know.] But labs (especially academic labs) are such cultural mixing bowls and (for better or for worse) we've arrived at a relatively judgment-less place in our workplaces; there's just not a lot of room for moralizing. Also, I'm not sure that anyone would take the ethical stylings of a young, 30 to 40-year old professor very seriously.
Perhaps later (much later?) in my career, I'd be comfortable in launching into a 10 minute sermon on the holiness of marriage and how they need to go home and beg forgiveness! right! now! But the safety perspective is um, safer, more relevant, and probably much less controversial.