I don't think that is an option? It's not in the gallery, and from what I remember Mark is exclusive to Holly if encouraged to go with him, or Lena can "steal" him from Holly if Holly's desire to be like Lena isn't high enough. I also don't recall any dialogue between Lena and Mark that wasn't explicitly explicit.
I haven't looked at the code, so perhaps
ffive or
Socrambus could give you more insight.
Did you perhaps mean Robert? I know that option was teased if Lena shared him with Holly and then she hooked up with him after they get back from the beach.