Yes homosexuality is psychological thing! What a suprise. Big plot twist coming ==> heterosexuality is also a psychological thing! hence its not natural !
Now some explanation but point is above
Actually if you go this way you will reach impulses, instinct levels that have at least some bilogical part (scientists have big problems in making strickt rules from what comes from bilogical formed body and what from enviroment and our own adaptation choices classy nature vs nurture problem). But lets claim that some basic impusles have mainly nature component. Then we can find that unformed human have both heterosexual and homosexual impulses. So in this case both are natural but !!! My point now !!! People measure their own impulses within time and they DECIDE to be one or another. So its a matter of choice not a concious one, with some biological background but its a choice. Many people have problems in measuring their own impulses and have problems with establishing their sexual orientation which prooves that its not 100% to be hetero, homo, asexual or sth but in this process there is nothing that promote one orientation over another.