The following discussion is only about biologically and mentally healthy individuals.

Sexual orientation is considered “complex behavior”.  It is influenced and shaped by many factors including but not limited to genes and environment.

According to APA (American Psychological Association):

Sexual orientation is defined as an often enduring pattern of emotional, romantic and/or sexual attractions of men to women or women to men (heterosexual), of women to women or men to men (homosexual), or by men or women to both sexes (bisexual).

Sexual orientation ie., who someone is attracted to may not be a choice but does it mean that the person loses control over whom he should have sexual relation with?

The main source of confusion among the public arises when the genetic expression of phenotype e.g., eye color, height, skin tone etc. is mixed with influence of genes on behaviors and choices a person makes.

A very popular answer to the question whether a “gay relationship” is a choice or not is that there are “gay genes” which predispose people to become homosexual with the presumption that predisposition equates to compulsion. The “gay gene” notion was mainly the result of the following study: 

In 1993, Dean Hamerand colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families. [27] Hamer et al. found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. In another finding, thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly dubbed the “gay gene” in the media, causing significant controversy.1 

Studies show that complex behavior like sexual and romantic relationship in no way could be attributed to one single factor as the study indicates: 

While our study results provide further evidence for early (pre-natal) biological influences on variation in male sexual orientation, we also emphasize that genetic contributions are far from determinant but instead represent a part of the trait’s multifactorial causation, both genetic and environmental.2  

As far as the studies done so far to discover the “gay gene” there is none discovered as yet. Another study in this regard states:

The findings, which are published on 29 August in Science and based on the genomes of nearly 500,000 people, shore up the results of earlier, smaller studies and confirm the suspicions of many scientists: while sexual preferences have a genetic component, no single gene has a large effect on sexual behaviors.

“There is no ‘gay gene’,” says lead study author Andrea Ganna, a geneticist at the Broad Institute of MIT and Harvard in Cambridge, Massachusetts.”3a

“We established that the underlying genetic architecture is highly complex; there is certainly no single genetic determinant (sometimes referred to as the “gay gene” in the media)”. 3b But there is always a possibility of choices turning into compulsions. Let us see an example of Alcoholism to see the role of genes in the disorder as it has been studied extensively. Drinking Alcohol for the first time is a choice which a person makes, to continue or not remains a choice till a person develops Alcohol use disorder after which he is somewhat helpless in continuing the behavior “alcohol use”. 

A large genomic study of nearly 275,000 people led by Penn Medicine researchers revealed new insights into genetic drivers of heavy drinking and alcohol use disorder (AUD), the uncontrollable pattern of alcohol use commonly referred to as alcoholism. In the largest-ever genome-wide association study (GWAS) of both traits in the same population, a team of researchers found 18 genetic variants of significance associated with either heavy alcohol consumption, AUD, or both. Interestingly, while five of the variants overlapped, eight were only associated with consumption and five with AUD only.
4

To conclude, as yet NO definite proof of a “gay gene” has been found and even if there is genetic predisposition it definitely does NOT compel certain behaviour by itself as Alcohol does not cause Alcohol use disorder even if a person has genetic predisposition, if a person doesn’t start drinking Alcohol in the first place.

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1. “Biology and sexual orientation,”  Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Biology_and_sexual_orientation&oldid=993167300 (accessed December 10, 2020).

2. “Genome-wide scan demonstrates significant linkage for male sexual orientation” Psychological Medicine (2015), 45, 1379–1388. © Cambridge University Press 2014 doi:10.1017/S0033291714002451 (accessed December 10, 2020).

3a. “No ‘gay gene’: study looks at genetic basis of sexuality. Five DNA markers are linked to sexual behaviour — but can’t predict a person’s preferences.” Nature 29 AUGUST 2019 https://www.nature.com/nature, https://doi.org/10.1038/d41586-019-02585-6  (accessed December 10, 2020).

3b. https://science.sciencemag.org, “Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior.” http://science.sciencemag.org/content/365/6456/eaat7693.full . Science 30 Aug 2019: (accessed December 10, 2020).

4. University of Pennsylvania School of Medicine. “Study reveals genes associated with heavy drinking and alcoholism: Unique genetic variants may inform future treatments for each alcohol disorder.” ScienceDaily. www.sciencedaily.com/releases/2019/04/190402124314.htm (accessed December 10, 2020).