Men, by nature is selfish and teritorial when we talk of "owning" a girl.. specially if dami umaaligid na mga guys sa girl "nya".. this doesnt necesarilly mena na mahal nya ang girl, in fairness though, most reasons are LOVE....
the reasons that i can think why men "wants" anal sex is that:
1. "papasukan ko lahat ng virgin sa kanya" - not contented with the normal way, wanted more, kung kasya lang cguro sa taenga at ilong ang shaft, g@g@win just to incorrectly own his teritory.
2. "mas masikp ang anus" - true for those girls that had been on the business for quite sometime
3. "maiba naman" - human, by nature love to explore
4. "taragis ka!" - may kasamang galit at selos, pinapahirapan ang girl
these are low hanging fruits that i can think of at this time.....
hope this helps.