Three questions:
1) As vadio said, is there no option for 'Spouse' or 'Partner'?
2) Are you definitely filling out the correct online application?
For a spousal visa, under visa type, you should have selected:
Settlement -> Settlement -> Wife
3) Are you aware that you also need to print out and flll in (by hand)
Appendix 2? This form also asks your relationship to the sponsor and it does have 'Spouse' as an option.
If you have the correct online form, and it doesn't have an option for Spouse or Partner, but it does have an option for 'Other', you could tick that and then if it lets you, write 'Spouse' in the box, or you can just tick one of the options and then correct it in pen after you have submitted the application form and printed it out.