Kinda wish I had a clear gendered attraction and a more binary gender so I could come out by saying something like "I like guys, but in a straight way"
I feel for you there. Not knowing what to call yourself sucks. I had to convince myself to say screw labels for now,so I can stop worrying about what a label might mean and figure how I feel and then label it. Not sure where you're at on your journey, so you may be past that.
The terminology is confusing, so here's an easy way to remember it: List all men in one column, and all women in another column. If you can draw a relationship diagram using only straight lines between the columns, the relationship is "straight"; otherwise, it is probably gay (but might be straight, or something else, due to category boundary errors).
(NB: a relationship being "straight" does not imply that the relationship's participants are straight.)
At first glance, it might seem weird that we have special terminology for such an arbitrary subset of relationships, especially when it fails to line up with other senses of the word. However, "straight" relationships hold special ritual significance in many cultures, including the dominant cultures of most current and former colonial powers.
Yes, that's correct. One thing that might help you understand is that they're not merely declaring it, like Michael Scott declaring bankruptcy. Trans people are that gender and they simply either have finally realized it. The default assumption is usually that you simply are your assigned-at-birth gender, so it takes some time for trans people to realize that this assumption was wrong.
Since they are that gender, it's easier to see how they would be a straight relationship in your hypothetical, since they're simply now a woman dating men, which we all label as straight. Such terms look simply at the gender of the people.
Of course, these days, our understanding of gender is a lot more diverse than it used to be, which makes labels like gay and straight more complex. Eg, non-binary people are on the rise (or more accurately, I suspect knowing how to label it and being out is on the rise). How do you describe a relationship with a non-binary person? It highlights how labels like straight and gay are usually very rough labels.