is homosexuality (or a deviant orientation) a normal biological phenomenon or an abnormality ?

Answer

Your question contains a number of ambiguities. They need to be clarified first.
1. Biological phenomenon: do we limit ourselves to all animal species or to some animal species: e.g. the mammals? All mammals or not?
2. ‘Normal’: The term is sometimes used in a statistical sense and sometimes in a moral sense. Statistically: how many times does it occur? Moral: does it deviate from the usual norm?
Even with the distinction ‘statistical’ you still have a question: from as soon as the phenomenon ‘normal’ from as soon as it occurs once? Or should something occur with what average number in order to be statistically ‘normal’?
With morally ‘normal’: it may be that a behavior deviates from a norm, but that it is still tolerated. While – because it deviates from the norm – it is branded as ‘abnormal’ and not tolerated.
Added to this is the fact that some behaviors are ‘normal’ in ‘abnormal’ circumstances. That is, in ordinary, normal circumstances a behavior does not occur.
Eg. homosexual behavior in prisons, in exclusively male or female environments.
Personally, I have found that a male goose recently wanted to copulate with another male goose, perhaps because no female goose was present.

There are examples of other (male) mammals jumping on each other – perhaps in the absence of females….But can we call this ‘homosexual’ behaviour? In the literal, biological sense, yes: it concerns mating behavior between two male animals.
It is, however, anomalous insofar as it is not ordinary, ‘statistically normal’ behaviour. It is ‘deviating’ from the norm.

When one now looks at homosexual behavior in people, you have a lot of new problems. It occurs in some population groups (past and present), whether or not wanted by one of the partners, because there is no partner of the opposite sex ‘available’ yet. It is not considered ‘bad’ because either the man (because that is usually what it is about) is married or will get married later. It may be disapproved, but otherwise there is no heavy lifting. Because it is assumed that the man/boy in question is fundamentally ‘heterosexual’ oriented. In this case you could possibly speak of the intention of doing birth control. But there is – as far as I know – no reference to that.

In a number of Western countries, (male) homosexuality (also female homosexuality, but less serious about it) occurs in two forms:
Or as experimental behavior, for fun, among friends, without fundamentally targeting someone of the same sex.
Or as a fundamental orientation. In the past, this was condemned as ‘deviant’. For a number of years, a certain social tolerance has developed (although not yet for 100%), which means that even the legislation of a number of countries has changed that allows LGBs (gay men and lesbian women) to marry each other and no longer may be discriminated against.

Scientists are not yet 100% sure whether this is a purely biological/physiological fact (in our DNA) or a psychological one (from upbringing, past,…). But suppose it is biological, traceable in our DNA: should we accept this or do we urgently need to find a remedy to repair this faulty (?) DNA. Then we are in a broader discussion about eugenics: adapting humans to so-called ‘ideal’ characteristics of the ‘ideal’ human…

Conclusion: Some people (most?) are deviant in appearance, behavior or lifestyle. (hair color, choice of clothing, homosexual behavior, handicap, protruding ears, bow-legs, giftedness, Down syndrome, low/high intelligence,….) . Where is the limit that a certain group will say that this deviation cannot be tolerated? Can the person in question choose to do so or not? Should that choice be sacrificed because the group will not tolerate it? And what if you didn’t choose it? Heterosexuality is also an orientation that you do not choose. The same goes for homosexuality for those who simply feel attracted to someone of their own sex.
It seems logical to me that society sets limits to protect the weak (cf. no sexual relations under a certain age because the partner cannot choose, or no sexual relations under duress). How far this should go is not yet clear. The case of the SM judge showed that people reacted very divided on this: for some, this is a free choice of both parties and therefore withdrawn from the judgment of others. For the others, this behavior was unacceptable and had to be condemned as ‘fornication’.

Hopefully this will provide enough material to continue the discussion in class.