Unmarried partner of a person with Ancestry visa? VAF1

Hi

I would like to apply as a Dependant of a person in a long-stay category but i need some help in filling out the VAF1 form. I am the unmarried partner of a guy applying for Ancestry. In question 2.6a Marital status I'm not sure which category applies to me? This is what it says on the form:

2.6a Marital status
Single  Married  Divorced  Widowed  Separated 
Civil Partnership 
2.6b If you selected "Civil Partnership", please confirm current status of relationship
Existing Civil Partnership  Dissolved Civil Partnership 
Surviving Civil Partner (Civil Partner deceased) 

There is no De facto relationship box and I believe civil partnership only relates to same sex partners who have had a civil partnership ceremony. We are a heterosexual couple and have been in a De facto relationship for 5 years.

Can anyone tell me which box to tick?

Cheers!