[Freesteyelz]

Yeah. You'll need to add the $comma to separate each selection. It's not that much of an edit but you'll probably want to replace what you have with:

<if condition="$comma = ''"></if>

<if condition="$post['field15'] & 1">
    $comma Serious Relationships
    <if condition="$comma = ', '"></if>
</if>
<if condition="$post['field15'] & 2">
    $comma Friends
    <if condition="$comma = ', '"></if>
</if>
<if condition="$post['field15'] & 4">
    $comma Networking
    <if condition="$comma = ', '"></if>
</if>
<if condition="$post['field15'] & 8">
    $comma Dating
    <if condition="$comma = ', '"></if>
</if>

See if that helps.