Karmic result of a gay man marrying a straight woman fraudulently
Fraudulent marriages may distinctly fall under "PishAcha" marriage (sub demonic), which is the lowest category among all types of marriages. According to Manusmruti, chapter 3, among 8 types of marriages, 2 of them, viz. Asura & PishAcha -- should never by practiced.
- When (a man) by stealth seduces a girl who is sleeping, intoxicated, or disordered in intellect, that is the eighth, the most base and sinful rite of the Pisakas.
- But in these (Institutes of the sacred law) three of the five (last) are declared to be lawful and two unlawful; the Paisaka and the Asura (rites) must never be used.
If a gay person, succeeds in seducing a girl, because her intellect couldn't help her find the reality, then he may fall under extremely sinful marriage as above.
Karmik punishment could be hellish treatment in present life or afterlife.
Having said above, the case which you described doesn't seem that sinful to me. Because the gay man is satisfying the straight woman's physical needs.
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