In a judgement regarding partner’s visa applications, handed down by the immigration upper tribunal in the UK; Fetle (Partners: two year requirement)  UKUT 00267 (IAC),
http://www.bailii.org/uk/cases/UKUT/IAC/2014/_UKUT_267_iac.html, the UK Immigration tribunal has stated that where “parties have been living together in a relationship akin to either a marriage or a civil partnership which has subsisted for two years or more”, partner’s visa applications such as those where you are stating that you have been with your partner for two years do not require two years cohabitation, but two years subsistence of the relationship.
Whether the relationship with your partner still subsists, as required by the tense of that requirement and as may be separately required, is a different issue in a UK partner’s visa application.
Following on from this judgement, when you make an application based on your relationship with your partner (same sex or otherwise), then the primary requirement will be to prove that your relationship has been in existence for a minimum period of two years.
In practice, we will always advise to provide two years proof of cohabitation in your partner’s visa application but where this is not possible, we can therefore use evidence of your relationship with your significant other, having subsisted for two years or more for your UK immigration applications.