is this an exmaple of the definist fallacy?

I've seen someone on a forum define the gay as: "The attraction to a person who posses either same-sex chromosomes or same-sex secondary characteristics"

Additionally, how would someone potentially argue against this if someone were to say "this is how I define the word"

I don't think it would be an argument - more of a mutual understanding to have a discussion about the topic in goodwill. If one person is talking about attraction, and another is talking about the physical act itself, they are unlikely to make progress. So it is not a fallacy to say something like "this is how I understand the term to be defined..." As usual, intent plays a large part in fallacies (that are not deductive).

The key here is to understand that different definitions of terms do exist, and what is most important is that involved parties make an honest effort to make sure they are talking about the same phenomenon.

Yes, the example you've provided can be considered an instance of the definist fallacy. The definist fallacy occurs when someone defines a term in a way that is contentious or loaded, often to support a particular argument or viewpoint without proper justification.

In this case, the definition of "gay" as "the attraction to a person who possesses either same-sex chromosomes or same-sex secondary characteristics" attempts to frame the concept in a particular way that may not align with common usage or understanding. This definition might conflate various aspects of sexual orientation, biological sex, and gender identity, which are distinct and complex concepts.

To potentially argue against someone saying "this is how I define the word," one could point out:

1. **Common Usage**: Highlight how language conventionally defines terms based on societal, cultural, and linguistic contexts. The word "gay" is broadly understood to mean a person who is romantically or sexually attracted to people of the same gender. Definitions that deviate significantly from widely accepted meanings may lead to confusion and misunderstanding.

2. **Precision and Clarity**: Emphasize the importance of precision and clarity in definitions. A definition should capture the essential characteristics of a concept without being overly broad or narrow. The provided definition might be too broad and ambiguous, as it could include scenarios that do not align with how "gay" is commonly understood.

3. **Avoiding Loaded Language**: Point out the potential for loaded language and ensure that definitions do not carry implicit assumptions or value judgments without evidence. A definition should be neutral and descriptive rather than prescriptive or biased.

4. **Consistency with Related Concepts**: Discuss how related concepts, like sexual orientation, gender identity, and biological sex, should be clearly distinguished. A good definition should be consistent with and make distinctions among related concepts to avoid conflating different ideas.

5. **Linguistic Authority**: Consider relying on linguistic authorities, such as reputable dictionaries or academic sources, which provide definitions based on standard usage and scholarly consensus. This can help ground arguments in widely accepted understandings of a term.

Overall, while individuals can have personal interpretations of words, effective communication relies on shared understandings of language. Disputes over definitions should aim to promote clarity and mutual comprehension.

Without seeing the argument itself it's hard to tell, but this does seem likely to commit the definist fallacy.

But, one problem with hinging arguments on definitions, just generally, is that every definition was made up by someone to describe or categorize something, and as such, there can be some legitimate disagreement about how things are defined, particularly as our understanding grows. For instance, prior to the invention of the digital camera, the definition of photography would been written to describe the photochemical process that's used in things like film. Likewise, with sexual orientation, our understanding of that has changed as well. Even if you accept the gender binary as immutable, the definition given could also encompass people we might describe as bi-curious, for instance.

But the one issue I see with this specific definition in chromosomes. People aren't doing genetic testing on people before they decide whether to be attracted to them. That starts to get blurry when you think about intersex people, some of whom don't have XX or XY chromosomes, but are virtually indistinguishable from those that do.