The 32-second advert, titled 'Dare to Change', was made for Wuaki.tv, a provider of on-demand internet streaming media, by branding agency Commo!
A brief scene in which two men are seen kissing led to a complaint from a viewer to the National Commission for Markets and Competition (CNMC) which is being investigated by Autocontrol, the self-regulation body of Spain's advertising industry.
News of the investigation was met with disbelief by LGBT groups including Catalonia's Gay Liberation Front (FAGC) whose president, Eugeni Rodríguez, described it as "homophobic", according to Catalan daily El Periodico.
The ad campaign, he said, "deserved the recognition of everyone," as an example of "promoting tolerance and equality" instead of being "in the gun sights" of regulators.
"Can anyone imagine a complaint about showing a kiss between a man and a woman? A basic gesture of affection between two people, whatever their gender, should not be the subject of an official complaint," he added.
Josep Monleón, content director for Wuaki.tv, said, "We were mortified to learn that, in the 21st century, we should have to justify that scene because someone thinks is it worthy of a complaint."
"We only wanted to show scenes that exemplified changes in mentality. We have serious doubts that the prejudice demonstrated by the individual who made the complaint would have been triggered by a heterosexual scene," he added.
Rodríguez questioned the CNMCs system of handling complaints, saying, "That they should accept this and pass it on to Autocontrol to be investigated is not just sad, it's intolerable."
He said that he would urge LGBT groups to promote the advert and political parties to raise a question in parliament about the CNMC's reaction.
Autocontrol, a self-regulation body of the advertising industry to which Wuaki.tv does not belong, said that it had to investigate "any complaint" it received but refused to make further statements.