Plant homosexuality is an interesting idea although most of the articles dealing with the concept have little scientific merit and don't corroborate the claims of plant homosexuality. As a rule, they conflate the botanical descriptions of plants as bisexual—a description of the biological sex(es) of an organism—with the colloquial usage which regards sexual orientation. Without resorting to an ad hominem argument, the fact that three of the four sources cited in the writeup above use posited 'plant homosexuality' to rationalize homosexual behavior in humans as natural demonstrates this.
Scientifically, there are several factors which confound attempts to classify plant behaviors into conventional sexualities. Around 90% of flowering plants are hermaphroditic (also called bisexual by botanists), having both male and female reproductive organs. The difficulty of classifying plants by sex makes it enormously more difficult to discern whether sexual interaction between these plants is homosexual or one of sexual competition.
Because of this, definitively claiming homosexual behavior in plants necessarily requires that they both be unisexual plants of the same gender. The example given above, of a plant selectively spreading its spores in the direction of another male plant, can potentially be interpreted as either genuine intent (insofar as a plant can have intent) or competition. For the latter, a plant may gain reproductive advantage by covering another male plant with spores. By doing this the first plant increases its chances of having its spores spread by contact with pollinators since its pollen is now spread over a larger area. Additionally the layering of the aggressor's spores on top of the spores of the other plant might act as a barrier to the spread of the second plant's spores.
Alternatively, the same act could also function as a form of intraspecific symbiosis. Many plants, most notably bamboos, have mass flowerings where all the plants in a species flower, reproduce, and die at the same time. While this may decrease the reproductive success of an individual plant, on a species-wide basis it can prove beneficial, resulting in so many offspring that predators simply can't eat them all. On a smaller scale, a plant mixing its spores with that of another plant may raise the density of pollen in that particular airstream, increasing the chances of successful pollination further down the line. This doesn't present an advantage to either plant but the species as a whole would benefit.
Other than unisexual plants, which are only one sex for life, there are more transient forms of single-sex plants. Protoandry and protogyny are complementary classifications where the sex organs of a plant are produced in succession. This chronological separation is used by plants to prevent self-pollination and inbreeding. Thus, a temporarily male plant could select another 'male' plant knowing that female parts will develop later and be pollinated by any spores remaining on it. This, again, is heterosexual behavior.
And even among truly hermaphroditic plants, there are explanations for apparently sub-optimal sexual selection. A plant with mostly male flowers could 'choose' another plant with mostly male flowers. I would argue that it would be hard to call this homosexual behavior in any context (since there will always flowers of the opposite sex present on each plant). But even if a 'male' plant specifically targeted another 'male' plant, it can easily be explained by signaling rather than a sexual orientation. It's easy to imagine that a high male to female flower ratio is a way for the plant to signal fitness that we don't yet understand. Perhaps a form of Fisherian runaway where the use of energy to produce superfluous male flowers represents that the plant is so fit that it is able to waste energy in a conspicuous way. A more in depth discussion of this particular scenario is here.
On the other hand, it should be noted that there are also some potential explanations for genuine homosexual behavior by plants. There is, of course, the 'born this way' argument—which works equally well with plants as with animals. A mutation in the genes that encode responses to pheromones could explain such behavior. Or environmental factors including pollution which could affect gene expression and plant development in other ways.
It also bears mentioning that a homosexual interaction, while not resulting in progeny, may increase the fitness of surrounding or related individuals. In humans it has been observed that the female relatives of gay males tend to have more children on average, seeming to indicate that the genetic 'dead ends' of homosexual sons is somewhat offset by the increased fecundity of daughters. There's no reason to believe that a similar relationship can't be found in plants.
All these phenomena indicate the wide diversity in modes of sexual reproduction and how very simple stimuli can lead to complex interactions. But describing these interactions in terms of human sexuality, especially for a form of life so fundamentally different from us, seems a stretch. More likely it's simply an attempt by a minority to defang the arguments of their critics.