An I obligated to attend this workshop?
I suppose at it's ultimate distillation no you're not - you aren't a slave or indentured servant and you can always vote with your feet if you find a particular job duty to be too onerous. However if you're asking whether they can require you to attend as a condition of your ongoing employment then there's every chance that they can. Of course if you do kick up a fuss about attending they might let you out anyway, but this is unlikely to be consequence-free, your employer and/or colleagues may well draw unfavourable conclusions about you.
I know you cannot discriminate against people for their race, religion or sexual orientation
Spot on! That doesn't mean that you won't learn something valuable in this workshop however, because I doubt that this workshop is just telling you this (after all a simple one-liner e-mail covers that) but it may well go some way to clarifying how this should be interpreted and demonstrating practical ways you can follow this.
There's a strong argument that precisely as someone whose interests, beliefs, and religion don't align with LGBTQ+ matters you are more likely to find yourself in the position of making an inadvertent gaffe and landing yourself in trouble. Since having to spend a great deal of time and effort explaining that you weren't meaning to discriminate against a group after the fact is tedious and unpleasant why not proactively engage in ways to protect yourself?
I feel like this is reverse discrimination in making me have to attend this as I have no interest in it
Being disinterested, or even bored isn't a protected characteristic anywhere as far as I know. I find myself doing things all the time at work that have no particular interest in, one of the reasons people get paid to go to work is as a tacit "we want you to do activity X when you'd much rather be doing activity Y, here's come money to compensate you for your time" deal. Me? I find safety briefings about as interesting as a year old copy of the phone book but I understand that the potential consequences of me doing something wrong because I didn't attend, or pay attention to, them could be exceedingly unpleasant for me, my colleagues, and my employer. So I damn well turn up and I damn well pay attention!
Unless you've been singled out simply for having (not acting on) personal or religious beliefs on the the subject I'm not seeing any way this is discrimination (reverse or otherwise).
...do not think it is my employers right to tell me how to think
It's not their right, they're entitled (within reason) to put forth a way they would like you to think, and you're entitled not to. It is however their right to tell you how to behave (within certain bounds of course), so you're free to think however you like about LGBTQ+ people and issues, but you aren't free to act in certain ways in your professional life, which you already know.
it doesn't align with my personal or religious beliefs.
As regards the religious aspect, I'm not aware of any major religions that prohibit their adherants from attending an LGBTQ+ related workshop, typically they prohibit their adherants from being LGBTQ+ instead, but my knowledge isn't complete by any means. If your religion does happen to have this, oddly specific, tenet you can probably excuse yourself with some supporting evidence as to that being the case, if you're unsure as to the position of your faith then I'm sure there's religious authorities you can consult.