Maybe the solution is to stop giving these degrees out to politicians of any stripe, & thereby avoid having to tie yourself into mental knots justifying why you draw the line where you want to do it. 

I didn't vote for either President Obama or President Trump; but once you justify giving the honorarium to one politician who doesn't accord full square with Catholic teaching, I think it's hard to justify drawing the line somewhere else the next time no matter how odious you find the next politician's positions. Perhaps, like the decision to cancel the White House Correspondents Dinner, it is time for this practice to come to an end as well. 

Another solution is to follow the practice of the Ivies, & separate the speaker from the actual commencement event. That way the commencement event remains genuinely focused on the students, & you can choose separately whether to participate in the class day event with the speaker.