The ad, created by First Choice Media, said in large lettering "Want longer lasting sex?" with the word "sex" in extra large lettering.
It was followed by smaller text stating "Nasal delivery technology. Call the doctors at Advanced Medical Institute."
The ASA received 552 complaints from people who believed the poster was offensive and therefore unsuitable for display in public locations, which included near schools and in areas with a high Jewish population, where it could be seen by children.
The advertising watchdog also challenged whether the poster advertised an unlicensed medicine.
AMI pointed out that it had complied with the ASA's request to withdraw all posters pending the outcome of the investigation.
It defended the campaign explaining that it was trying to address serious men's health issues while removing the stigma and taboo associated with asking for help.
It said that talking about sexual problems in an open manner is not irresponsible and pointed out that frank discussions about people's sexual problems have been present in the media for decades.
AMI said we were "living in a more liberal and tolerant age than, for example, 10 years ago". It recognised a number of complaints had been received but believed in the wider context of reality TV programmes, lads' mags, online content and prime-time TV shows such as Channel 4's 'Embarrassing Illnesses/Bodies'.
It highlighted the recent high-profile poster campaign for the 2008 film 'Sex and the City' which emblazoned the word "sex" in very large lettering all over the UK.
The ASA upheld the complaints and said that the poster was unsuitable for public display.
It noted AMI's argument about delivering a message but also noted that a number of people who had seen the posters had felt that the language used was offensive and inappropriate for general public display.
It also said that many people considered the posters' bright colours and very large text, including the word "sex" to attract attention, was unsubtle and crass.
In response to the challenge about whether the poster advertised an unlicensed medicine AMI said that it believed that it had not promoted a medicine or referred to any medication but rather advertised a delivery system relating to treatment.
The advertising watchdog upheld this complaint as well saying that by referring to nasal delivery AMI had indirectly advertised the medicine itself.
It noted that the AMI did not hold marketing authorisation for any medicines prescribed as part of their treatment programmes.