my parents think its normal to attend wedding ceremonies theyre not invited to. is it?

I think that attending a ceremony is fine but never a reception.

Many people attend the wedding ceremony who are not invited, but they dont go to the reception

I would not think it usual to attend a ceremony to which I have not been invited. There is no rule but i guess you could contact the couple and say how  fond you are of them and would like to be part of their celebration.

It may depend on your parent's culture and the culture of the couple getting married. For example in the Dutch culture anyone who knows the couple from work or socially can attend the church wedding ceremony and can attend what they call the reception which is drinks and nibbles but only family and close friends attend the wedding dinner or what we call the reception.

Most commonly in Australia people should wait to be invited to a wedding due to the way we organise weddings and the associated restrictions and costs.

Marry Me Marilyn

Hello

I don't see the problem with attending a wedding that you have not been invited to. Weddings are expensive events and families cannot always afford to invite everyone, but are happy for everyone to attend the wedding service and selected guests to attend the celebrtion after. 

Hi there, 
Whilst it's not "the norm" it is not uncommon either. 
Etiiquette would say wait for the invite. Many people have ceremnoy only invites as they have limited space at the reception, however the same is also true of ceremony space, somethimes it too is limited. 
Cx

They should wait for the invitation. Some couples have limited spaces for the ceremonies. Some couples have more invitations for the receptions and not necessary for the ceremonies. Some couples even elope for the ceremony with just have 2 witnesses.