Thanks for the question. This is certainly a sticky issue. I personally have no problem marrying two nonbelievers together, since they are not unequally yoked and this is a civil duty for ministers. I view this similar to conducting a funeral for an unbeliever. If those getting married (or getting buried) are unsaved, then most of their guests will also be unsaved, and this provides a great opportunity for the Gospel--as you mention. I have drawn the line, however, if the unbelieving couple (or family of the deceased for a funeral) ask me not to talk about Jesus. At that point, I have refused the opportunity.