Whether and how to thank a girlfriend or boyfriend in the acknowledgements of thesis?
The acknowledgements section is really yours to do with as you wish.  You can thank anybody who has been of help and support to you, no matter how formal or informal your relationship with them is.  You can make the relationship explicit or not, however you prefer.  Thus, for example, it is equally valid to write:


  Thank you to Jane Smith, for all her love and support


or 


  Thank you to my girlfriend Jane Smith, for all her love and support