I am sure there isn’t lesbianism among any animals.
I wouldn’t want to anthropomorphise non-humans by using the term “lesbianism”, but sexual activity between females is definitely observed in bonobos, macaques, marmots, cattle, and lions.
Male to male only happens to mamals in captivity in female absence.
Not true. It is observed in the wild. It is even observed in domesticated livestock (especially sheep) where the males are positively encouraged to mate with females.
Sex taken out of reproductive purpose completely becomes one of mere entertainment purpose.
That is my point. In humans and non-humans sexual activity serves purposes other than reproduction. You may not approve of this morally, but it is a demonstrable fact that it occurs. Humans participate in a range of activities that involve the male ejaculating outside the female’s vagina. Humans also participate in activities that are oriented solely towards the female achieving orgasm. You may think that that is immoral, but you cannot say that it does not take place. You also cannot say that it does not occur in non-human species. It may serve a number of ends, e.g. pleasure, social bonding, and establishing social dominance.
Each biology process has a purpose.
A biological process may serve more than one purpose. Humans (and some other species), for example, commonly have sexual intercourse during pregnancy when the likelihood of the female becoming pregnant again is negligible. It is argued that the benefit of this is bonding the couple and ensuring that the male stays with the female to help care for the offspring instead of breeding with other females.
Your lung analogies are irrelevant. Of course nobody should have healthy lung tissue removed and of course a pump isn’t a lung.
More interesting would be to ask why animals eat different kinds of foods. It’s not just to acquire necessary nutrition from those foods. Many species can survive on a much more limited range of foods than they choose to eat. Seemingly animals are curious about different foods and enjoy different foods. In that instance eating does not only serve the end of physical survival, but also pleasure and satisfying curiosity.
And if you really think that you need to explain sexual reproduction to me, perhaps you should be aware that it is not only mammals that reproduce sexually!