Formal Swedish title options:

• Legalisering av samkönade äktenskap i Thailand föranleder massbröllop. (Legalization of same-sex marriage in Thailand leads to mass wedding.) – This is probably the most straightforward and formally correct option.

Other slightly less formal but still appropriate options:

• Massbröllop i Thailand efter legalisering av samkönade äktenskap. (Mass wedding in Thailand after legalization of same-sex marriage.)