I have a couple of theories about 2 things in DOL which have been puzzling me:
1. Why does Mildmay dislike Murtagh on sight?
2. Why is it that Felix is so against 'doing women'?
In answer to the first question, I think that because Murtagh reminds Felix of Malkar, Mildmay must have a similar feeling of recognition too, maybe subconsciously, so he instinctively reacts to him the way he does, also considering he's never really worked out the issues of what was done to him while he was being captive.
Felix is forced to interact with Murtagh, and finds out that he's not like Malkar at all, so he's ok with him.
About the second question, I wonder if Felix feels so strongly about it because he does not want to risk fathering a child?
If he accidentally made some strange woman pregnant, in his mind (growing up in Melusine where so many children are abandoned to roam in feral packs or are sold to keepers/brothels) there is a distinct possibility his child could end up having a fate similar to his own, and considering the kind of childhood he had, he wouldn't want to wish it on his worst enemy.
I think most of it is the Malkar resemblance. But when Mildmay first meets Murtagh on the train, he and Felix are playing a bit with the flame/shadow thing they had, and I think Mildmay's hackles are raised by that. He's very protective of Felix.
So he looks like Malkar, and then he's teasing Felix, and he's a duke. He's pretty much struck out in Mildmay's eyes right there.