I think one of the themes in this piece is sacrifice.
This stranger seems to be travelling through the night on a weather-laden day. He asks the bridegroom for a place to stay for the night, but the bridegroom likely has plans for his woman this night.
So, he considers what he was willing to sacrifice:
These things a little food or money or even a prayer would be easy to give... and the bridegroom would be willing to give them. The sacrifice of a few bucks far outweighed the sacrifice of this night he has possibly waited for a long time. The ultimate question came when he considered:
The bridegroom thought it little to give A dole of bread, a purse, A heartfelt prayer for the poor of God, Or for the rich a curse;
But whether or not a man was asked To mar the love of two By harboring woe in the bridal house, The bridegroom wished he knew.He knew if he brought this man in, he would do so unwillingly and he would be disgruntled about it. It would be killing his chance to express his love the way he wants to on this night. That is not something he wants to sacrifice. Other themes to consider might be decision-making, or types of love.