I wouldn't call that cowardly at all. That's the way I did it.
I have social anxiety issues, so I can't do confrontation. I slowly changed my clothing bit by bit (this was probably less obvious from my end as I'm FtM and I was already considered a tomboy.) Gender-neutral clothes to blatantly boy clothes, eventually cutting my hair, buying boxers and men's deoderant, all while having carefully planned excuses for everything (ex. I would have told them my binder was a special kind of sports wear, and I only washed them at night.) I got to the point where I was tired of being in the closet, but I hoped I could make them guess rather than having to tell them. So I started attempting to be obvious with it. I left my binders out, I left my deoderant out, I tried subtle hints to see how they felt about trans people (ex. Asking how they felt when someone was on the news.)
Finally I got tired of waiting and decided they would never get the hint. So I waited for a day when I knew I'd be gone for at least 24hrs (to give them some time to process everything,) and I wrote them a letter. I then put the letter on the kitchen counter and left.
There are some people who would call that cowardly. If they want to think that, that's their business. I did it the only way I could. My anxiety issues would have made a face-to-face coming out pretty much impossible. Even if I could have managed it, chances are I would have been so nervous I would have stumbled over my words, unable to explain myself clearly, and forgotten half the things I wanted to say. I am much better at writing things out, so I did. I was able to organize my thoughts, explain everything to my own satisfaction, and not be in a situation that would probably have caused me a panic attack.
You have to do things the way that works for you. There are people out there who may call it cowardly, those people are judgemental and can't understand another person's point of view properly. Try not to get tangled up in that. It's not their place to judge you, they're not you and it's not their life.