I’m surprised Thailand with its 15 year old age of consent had not already approved “same sex marriage.”
Below 15, it is statutory rape. Period. End of story. If the girl or the girl's parents complain, you will be going to jail for a few years if she's below 18 (See Section 282 paragraph 2 of the Thai criminal code). There have been newsworthy cases where both Western and Thai men have been prosecuted and convicted for having sex with 16 or 17 year old girls.
15 would best be described as "marriageable age". 18 is the flat-out age of consent.