That's very sad about your friend...

How old is she? It's never too late, and it's not much harder to find a gay partner than a straight one unless you live in the middle of nowhere. Marriages generally have a 50% chance of ending in divorce, so it's not like getting married to a guy will ensure that she'll never be alone. Her being a lesbian + getting married out of fear = even higher chance of it falling apart. I hope she finds someone else who she's actually happy with.
I think some people can be happy without a relationship or children, but most people would feel lonely and are very family-orientated. If you're having trouble achieving these things, then focus on your self-development in the mean time. Make some goals and reach them, get a hobby, do interesting things, get fit, travel, make friendships, etc. This is a good path to start with because 1. it makes you more attractive as a partner and gives you more options, and 2. if you still never find a partner, you'll have other significant things in your life to bring you happiness.
Don't give into thinking that nobody wants you and you'll always be alone, because that attitude is probably 95% of the reason that you are currently single. There are plenty of people out there that won't care if you're trans. As for family, you have some (difficult but possible) options for having kids like adoption or surrogacy etc.