Just a thought.
Young male lions live together in "bachelor" groups until they are able to take control of a family of female lions. Breeding families consist of several females and a single male who does not tolerate other males. It is common for a new male, when taking over an existing family, to kill any cubs that are around. It is likely that the lions in the photograph are relatively young males.
My guess is that what is being seen is an act of hierarchical dominance between two members of a bachelor group.
I think that it is not necessarily accurate to describe this behaviour as "homosexual". In the case of most non-human animals sexual behaviour is a response which occurs because of the presence of particular stimuli - usually oestrus related. It is totally instinctive and related to reproduction - unlike the human sexual response which appears to be primarily concerned with pair-bonding rather than reproduction (but with reproduction being a consequence). Human sexual behaviour has a very large affective component which may manifest itself in same-sex relationships.
As such, they have become a component in human sexual behaviour. There is no reason to believe that lions develop such behaviour. My guess is that there may be females nearby who are in oestrus and their air-borne hormones are stimulating adult bachelor lions. One lion, being higher in the pecking (!) order than the other demonstrates his dominance in an attempt to respond to the hormonal stimulus he is experiencing.
