Fucking and fighting?  no.  but Spike forgot the final, essential part, the part they have in spades - loving.  With that added, then yes, I think they have enough.  More than most, in fact.

A wonderful new, but also very scary, new phase - and I can't help feeling just a little apprehensive with buffy's need to make that call so quickly. 
___________________________________________________

"He felt like a fool for ever doubting her, even as he wondered if fucking and fighting were enough to make a marriage on. Enough for her. "
___________________________________________________

It seems more like Spike is doubting his own part of the relationship - what he brings and gives to their relationship. 

Thank again for the new update - always a pleasure to see a new posting -

 

Ohmy, 'a sumptuous fuck'
only you can inspire such utter doubt about the beloveds' lives when Everything's good, wonderful right? Right, yeah (please?)? So, my stomach's in a knot now cos 'she'd allowed herself to trust'.
Miz Herself, this fabulous roller coaster ride of a story gives pleasure at every syllable--glad you update so lovingly.  Gawd, It's nice to be under the comfy covers again--