I am not one to minimize reasons for wanting to die, but I hope that you don't end up making your decision solely upon this. If you are able to be okay with his heterosexuality and just be friends with him, then go on this dinner with that expectation. If you will leave feeling further heartbroken and depressed, perhaps reconsider. There is no wrong answer here, it really all depends upon what you can handle.
In terms of giving yourself "another chance at life" I will always answer yes to anyone who asks. Though I am fully supportive of people who want to end their life, I genuinely do not believe it's the best first option. Or second. Or third. To me, it's a last resort type of thing. If you have even a shadow of a doubt about it, I would stay. If you think you can handle giving yourself a second chance in any capacity, do it.
IF this situation with your colleague is increasing your suicidal thoughts, please know that you can still live a fulfilling life despite the pain of this. Again, I do not want to dismiss the heartbreak you must be feeling but please don't let this situation factor into your decision.
Give yourself another shot. But do it for you.
