▁not ▁one - to - one . ▁After ▁their ▁mid - fort ies , ▁some ▁men ▁report ▁that ▁they ▁do ▁not ▁always ▁have ▁an ▁ere ction ▁when ▁they ▁are ▁sex ually ▁ar oused . ▁Equ ally , ▁a ▁male ▁ere ction ▁can ▁occur ▁during ▁sleep ▁( no ct urn al ▁pen ile ▁t umes c ence ) ▁without ▁conscious ▁sexual ▁ar ous al ▁or ▁due ▁to ▁mechanical ▁stim ulation ▁( e . g . ▁rub bing ▁against ▁the ▁bed ▁sheet ) ▁alone . ▁A ▁young ▁man   — ▁or ▁one ▁with ▁a ▁strong ▁lib ido   — ▁may ▁experience ▁enough ▁sexual ▁ar ous al ▁for ▁an ▁ere ction ▁to ▁result ▁from ▁a ▁passing ▁thought , ▁or ▁just ▁the ▁sight ▁of ▁a ▁passer by . ▁Once ▁ere ct , ▁his ▁pen is ▁may ▁gain ▁enough ▁stim ulation ▁from ▁contact ▁with ▁the ▁inside ▁of ▁his ▁cl othing ▁to ▁maintain ▁and ▁encou rage ▁it ▁for ▁some ▁time . ▁ ▁As ▁sexual ▁ar ous al ▁and ▁stim ulation ▁continues , ▁it ▁is ▁likely ▁that ▁the ▁gl ans ▁or ▁head ▁of ▁the ▁ere ct ▁pen is ▁will ▁sw ell ▁wider ▁and , ▁as ▁the ▁gen it als ▁become ▁further ▁eng org ed ▁with ▁blood , ▁their ▁color ▁deep ens ▁and ▁the ▁test icles ▁can ▁grow ▁up ▁to ▁ 5 0 % ▁larger . ▁As ▁the ▁test icles ▁continue ▁to ▁rise , ▁a ▁feeling ▁of ▁warm th ▁may ▁develop ▁around ▁them ▁and ▁the ▁per ine um . ▁With ▁further ▁sexual ▁stim ulation , ▁the ▁heart ▁rate ▁increases , ▁blood