In brief – a proposed law would have banned "conversion therapy" i.e., traditionally, the practice of trying to make someone be straight instead of gay. Which most liberal people would agree is a good thing to ban. It was/is usually practised by religious groups and was more common in the past.
However, the proposed law also applied to being trans, and many people saw that as a problem because if a young person identifies as trans, it's well-known that they often change their minds later, and there's a lot at stake as medication and surgery can cause irreversible changes. But the conversion therapy law when applied to this situation, would have meant a doctor or therapist would be forced to always "affirm" or agree the person was trans and give them medication/surgery if they wanted it. To even discuss possible reasons why they felt that way and/or consider other factors in them not feeling happy about their body (such as sexual abuse or trauma, or social contagion which is common in schools now) would be "conversion therapy" and so illegal.
There's also the issue that some kids who are gay have been known to be pushed towards being trans by parents who don't want a gay child, because that way they get a "straight" child. So in that way, a trans identity can actually be a kind of conversion therapy in itself.
If gay (LGB) and trans (T) had been dealt with separately, it wouldn't have been such a mess. They are very different things and involve different needs and there has been a lot of protest about the whole thing because of that – not because people are generally pro-conversion therapy.
There are various threads discussing it in detail as it's been under discussion for a few months.