if you really want to understand what's going on here, i would discourage you from entertaining the following notions any further:
watching transgendered porn makes him not straight or is somehow a problem for him or you
that men get auto-boners just from seeing a specific naked woman
your partner watching transgendered porn doesn't necessarily indicate that he desires a relationship with a transgendered person. futanari porn, for instance, is relatively popular with straight men. the encouraging thing is that he's open to discussing it with you, because not many straight men would admit that, let alone to their partner. rather than thinking about it as a problem, think about it as a route to new possibilities, like cam roleplaying, prostate play, or pegging. it's up to the two of you to decide how far you want to explore that curiosity together, but you'll be most successful if you don't view his kinks as an indictment of his straightness and/or masculinity.
as for the 2nd thing, i feel like a lot of women have this feeling about men and erections and i feel like it's based on a very cartoonish idea about male sexuality, one that is admittedly perpetuated mostly by straight men and the way straight men perform their sexuality.
there are so many factors that can affect any person's sexual performance, male or female: stress, sleep, diet, exercise, mental illness, physical illness, side effects of medication, not feeling sexy, lack of emotional intimacy, random circumstance, etc. then you've got to factor in everything about your relationship: how long you've been together, how well your sex drives match up, communication, how well you maintain intimacy in other areas of the relationship, whether or not you like the same things, etc.
in any prolonged relationship, there's going to be a drop-off in desire; no level of hotness can transcend biology. that's why communication, intimacy, creativity, and general game-ness are so important in long-term, monogamous relationships.