1) Just how big of an ego does she have to think that her show would be a platform for him...he's the President of the United States. Her show would benefit way more than he would by being there. He has access to billions of more viewers than she does.
2) I've never known Trump throughout his years of being in the public eye, to ever say or do anything against the LGBTQ community. He's a New Yorker. He's accepting. Maybe he's done something I don't know about but if Ellen wants perfection, she should take a look at some of the people she has had on her show who aren't good role models for anyone, including young, impressionable, girls.
She should reflect on a few of the public apologies she's had to make over the years. It's her choice on whether to have Trump as a guest (I don't like him), but she shouldn’t act like she’s taking the moral high ground by not having him on her show.