I would not do this in the Philippines. Anything other than a formal marriage with somebody below the age of 18 is definitely not recommended in that country.

In Cambodia, sex with somebody below the age of 18 would almost certainly involve aspects of trafficking or prostitution, as for normal girls, it is almost impossible to make arrangements without the mother. Cambodian girls also are the least sexuality-driven ones in all of Asia.

A bad idea for Vietnam, as you would be framed by various sides, including possibly the police.

In Indonesia, whether the girl is below or above 18, she or her family could always file charges if you don't marry her.

In Malaysia, the setting described in the headline is very difficult to achieve. If the girl is a Muslim, she would face jail and shame for sex outside marriage, regardless of her age.

Your best bet would be Thailand, as sexual relationships outside a prostitution setting are legal from the age of 15. It is not difficult in principle to approach girls, even when their age is below 18. Before you can enter a sexual relationship, a non-formal marriage would likely be requested. It is customary, for Thais and for foreigners, that a dowry of about 5000 US dollars is paid to the parents. The rather steep price does assure a certain level of sincerity on the part of the man, even if no genuine marriage is involved.

Please note: such informal marriages (from a legal perspective, these are just girlfriend relationships) are common among Thais. They often last only a few years, but could also just last merely a few weeks.