Hi there,
While I can't guarantee that the presence of cis/straight folks would prevent the others from expressing themselves, I also can't guarantee that it wouldn't prevent the others from expressing themselves.
There are also, many game jams not specifically for LGBTQIA+ individuals and I strongly advise you seek those out should this one not be organised to your liking or perhaps, you can organise an LGBTQIA+ game jam that invites cis/straight folks as well. I can certainly advise and support you in that organisational endeavour if you'd like :)
For now, this one remains for those that identify as LGBTQIA+.
Many thanks for your comment