Until later in the series, it seems like Mallory has a different boyfriend (or at least love interest) in each episode.  There are (for instance) episodes during the "Nick years" where Nick doesn't appear, but presumably they are still together.

Some have no specific timeframe.  For example, Steven remembers Walter Murchison in the episode "Mr. Wrong".

Episode Boyfriend
Pilot Richard DeFazio
Not with My Sister You Don't Eric Morrison
Big Brother is Watching Ralph Rappaport
Death of a Grocer Brad Hunter
A Christmas Story Donny Hayes
Sherry Baby Irwin "Skippy" Handelman (against her wishes)

French Lessons

The Fifth Wheel

Jeff Wakefield
Tender is the Knight some other boy Steven is worried about (Jeff is away at Princeton, as we see in the next episode)

The Homecoming

Go Tigers

Jeff Wakefield
Ready or Not Rick Harmon
Diary of a Young Girl Kevin Jenkins
Love Thy Neighbor Scott Handley (though Jennifer hates the idea)
Don't Kiss Me, I'm Only the Messenger Mallory plants a huge kiss on Skippy (in an attempt to spare his feelings about Jane)

Mr. Wrong

The Old College Try

Mr. Right

You've Got a Friend

Fool for Love

Engine Trouble

Art Lover

Paper Chase

Mrs. Wrong

The Big Fix

High School Confidential

Higher Love

Architect's Apprentice

A Tale of Two Cities

Keaton vs. Keaton

A, My Name is Alex

D is for Date

Dear Mallory

The Other Woman

The American Family

Miracle in Columbus

The Play's the Thing

The Spirit of Columbus

Read It and Weep

Spring Reminds Me

Father, Can You Spare a Dime?

It Happened One Night


Beyond Therapy


Deja Vu

Nick's Best Friend

The Wrecker's Ball

'Til Her Daddy Takes the T-Bird Away

Wrap Around the Clock

Mr. Keaton Takes a Vacation

Alex Doesn't Live Here Anymore

Nick Moore
Higher Love Mallory and Colin Spencer are attracted to one another, though Mallory is still in a relationship with Nick.
Matchmaker (filmed out of sequence) Roger Sloate
Anniversary Waltz (filmed out of sequence)

Kurt McMillan (a love interest)

Johnny Holmestead

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