npcs will sometimes develop a romantic relationship immediately after introduction. However it doesn't take into consideration the sims sexuality or/and if they have a romance bar with someone else. This happens in the base game but it's more obvious with Lovestruck installed as you can see the romantic satisfaction meter develop real time.
Iris Ladipo, a lesbian sim, was greeted by Oberon Summerdream, who is married, both developed a 50 friendship and 40 romance bar. I've noticed this only happens between a male and female sim and is caused by the loot action:
0C772E27!00000000!0000000000017DC0.Loot_Relationship_NPC_Initial_Social.ActionTuning.
Edit By Crinrict: Base game doesn't need a tag.
