Some English articles have categorised the series as a "gay drama" but many fans reject such simplistic labelling. It is also frequently lumped together with the much older "Boy's Love" or BL genre (previously called yaoi) of comics geared towards female readers featuring erotic relationships between men, though this is similarly rejected by fans who point out that a kiss scene is as physical as it gets in Ossan's Love. In fact, the word gay (ゲイ) - which usually refers only to male homosexuals in Japan - is never used in the show and it is never clear if the characters are homosexual (though there is one "coming out" scene where one character tells his father he likes men). Indeed, a central theme is one of moving beyond sexuality, reminiscent of the Foucauldian notion that sexuality - the classification of people into gay, straight, bisexual etc - is a relatively recent invention. The key message of the series is how wonderful it is to love someone - hito o suki ni naru koto (人を好きになること)- regardless of sex or age (the series also features a relationship between a young man and an older woman). Given this premise, it is rather surprising that there are no female-female relationships in the series; however, on second thoughts, maybe it is not so surprising given the invisibility of lesbian women in Japanese society (except in the form of male porn) - men tend to dominate Japanese LGBT movies.
The problem is that even among young people, there is little understanding of exactly what LGBT means. When I cover such issues in class - and I start off by focusing on LGB only - it is clear that most students don't know the difference between gender (how "masculinity" and "femininity" are constructed in a particular society) and sexuality (individual sexual preference). A big reason for this is that in Japan, sexuality is usually left unspecified (and unacknowledged): it is conflated with (and often subsumed under) gender. Here, the focus is on gender ambiguity (androgyny) rather than sexuality: transgression of gender boundaries (such as cross-dressing or josō=女装) rather then sexual preference. For example, Japanese words which are commonly translated into English as "gay", such as okama (おかま) and o-nē (オネー) - literally elder sister - actually denote individuals who act/dress/speak in a feminine way though they may - or may not - be gay. These ideas are explored in more detail in a great chapter by James Valentine available here. But if that is too heavy for you, you can simply enjoy the Ossan's Love film - or binge watch the series on Netflix (with English subtitles).