Suppose you feel that you live in a society where being gay or atheist is looked down upon. Moreover, suppose at the same time that you are gay or atheist and not ashamed of that anymore. Then, it makes sense that you'd want your 'validity recognized' from people close to you. Besides that, I can see many such people making a wider argument to others that they should accept that being gay or atheist is a valid choice.

I personally think that, besides the above, a lot of the loudness comes from a feeling of freedom. Where all of a sudden you no longer need to hide who you are, which leads to a stronger expression of a part of you that was pent-up.