The answer given by His Eminence, Ayatollah Mahdi Hadavi Tehrani (may Allah grant him long life) is as follows:
1. Any kind of relations between the opposite genders that entail evil and sins such as lustful look or physical contact are haram except for when they are Mahram to each other.
2. It is haram for two individuals from the opposite gender to seek any kind of pleasures from each other except through temporary or permanent marriage.
3. There is no problem in the friendship between two opposite genders, if it does not entail any evil and if there is no likelihood of falling into a sin.
For further details, see the following indexes:
1. Islam and Healthy Boy -Girl Relationship, question No.1044 (site:1110).
2. Allowable Sexual Relations between a Boy and a Girl, question No.713 (site: 754).
3. Boy – Girl Relations, question No.6669 (site: 6745)
4. Marriage; the Only to Satisfaction of Sexual Instinct, question No.1485 (site:1491)