Yes it is a dominance thing. I would correct it. My experience with poms made me think they are very independant & dominant. Tilula my yorkie will try to hump my husband maybe because she sees/hears me boss him around, she will bark at him, do the little pinch bites on his arm & then hump him pretty much saying you are my $!@#%
It is holarious. But I correct her because I know it will lead to more bad habits so definately stop it & be stern because it will lower your yorkies self worth & esteem plus the pom will keep finding more ways to make himself feel dominant & you will not want him eventually when it gets to far. You can use belly bands to help with marking & usually in a few days they will stop. But even though they have something on still say no when they lift their leg then say go outside, take them out/ belly band off & let them mark away. Well good luck.