Short answer:

• For monks and nuns (mahatmas): No. Complete celibacy (brahmacharya) in mind, speech, and body is mandatory.
• For lay Jains (shravaks/shravikas): Limited permission with strict restraints. Sex is allowed only within marriage, with one partner, and with self-control. Adultery, premarital/extramarital relations, coercion, abuse, or indulgent/excessive behavior are not permitted.

Why:

• Brahmacharya is one of the five great vows (mahavratas) for ascetics—absolute celibacy.
• For householders it is an anuvrata (small vow): fidelity to one spouse, avoidance of all other sexual contact, and cultivating restraint to reduce passions (kashayas).

Common guidelines for lay Jains:

• Be faithful to one spouse; avoid all other relations.
• Act with consent, care, and non‑violence; never harm, exploit, or coerce.
• Practice moderation; avoid stimulants of passion, pornography, and night‑time indulgence.
• Observe periodic abstinence on holy days (poshad/upvas) to strengthen self-control.
• Abortion and any act that destroys conceived life is violence and discouraged; methods that prevent conception before life begins are treated with caution and restraint.

Both Digambar and Shvetambar traditions agree on:

• Absolute celibacy for ascetics.
• Restricted, faithful, and non-violent conduct for laypeople aimed at spiritual progress.

If you want a simple summary: Monks—no sex at all. Household Jains—sex only with your spouse, with restraint and responsibility, as a step toward greater self-control.