Red Flag, if you are there they can "pretend they are not bi?" Who cares if they are bi and would that create an unhealthy dynmaic for you?
But after that here is my 2 cents / food for thought:
Do you think you would enjoy a 2 some with 2 men? Do you enjoy pegging and would the interaction between the two men be a turn on for you?
I think you need to clearly define what you would like to get out of this sexual experience from the men as well. Establish ground rules and engage in safer sex.
If all three of you are consenting adults and can enjoy one another why not?!
Strangely, why would he wants you you use a strap on harness when there are already 2 dicks in existence? Are 2 guys talking or implying pegging on them strictly?
Sounds very hot to me - but that's talking as a man with a bi side. If it's not appealing to you now then either you're undecided because it's a new thing or you're not keen for whatever reason.
If you're not keen, say no. If you're undecided, think about it a little bit. Is it something that might interest you? Would you be interested in some DP or a spit roast for yourself?